Update size guidance for iconImageUrl (SharePoint#811)

It actually displays at 40x28. We should update docs for this. 

SharePoint#703

Author: Nicholas Pape

docs/spfx/web-parts/guidance/simplify-adding-web-parts-with-preconfigured-entries.md

@@ -46,7 +46,7 @@ Property name           |Value type      |Required|Purpose
 title                   |ILocalizedString|yes     |The web part title that is displayed in the toolbox.              |`"title": { "default": "Weather", "nl-nl": "Weerbericht" }`
 description             |ILocalizedString|yes     |The web part description that is displayed in the toolbox tooltips.|`"description": { "default": "Shows weather in the given ___location", "nl-nl": "Toont weerbericht voor de opgegeven locatie" } `
 officeFabricIconFontName|string          |no      |The icon for the web part that is displayed in the toolbox. Its value must be one of the [Office UI Fabric icon names](https://dev.office.com/fabric#/styles/icons). If this property has a value, the **iconImageUrl** property will be ignored.|`"officeFabricIconFontName": "Sunny"`
-iconImageUrl            |string          |no      |The icon for the web part that is displayed in the toolbox and is represented by an image URL. The image at the URL must be exactly 38 x 38 px. If the **officeFabricIconName** property does not have a value, this property must have a value.|`"iconImageUrl": "https://cdn.contoso.com/weather.png"`
+iconImageUrl            |string          |no      |The icon for the web part that is displayed in the toolbox and is represented by an image URL. The image at the URL must be exactly 40 x 28 px. If the **officeFabricIconName** property does not have a value, this property must have a value.|`"iconImageUrl": "https://cdn.contoso.com/weather.png"`
 groupId                 |string          |yes     |The group id determines which toolbox group will contain the web part. The SharePoint Framework reserves group ids for default groups. The developer can pick one of those groups. If a group id is specified, then the **group** property will be ignored. Alternatively, the developer can pick a completely unique id and a group name. The toolbox will then show the web part in its own group.|`"groupId": "6737645a-4443-4210-a70e-e5e2a219133a"`
 group                   |ILocalizedString|no      |The name of the group in the toolbox in which the web part will be displayed. If no value is provided, then the web part will be displayed in the **Custom** group.|`"group": { "default": "Content", "nl-nl": "Inhoud" }`
 dataVersion             |string          |no      |Use this field to specify the data version of the pre-configured data provided to the web part. Note that data version is different from the version field in the manifest. The manifest version is used to control the versioning of the web part code, while data version is used to control the versioning of the serialized data of the web part. Refer to dataVersion field of your web part for more information. Supported values format: MAJOR.MINOR version|`"dataVersion": "1.0"`
@@ -538,4 +538,4 @@ gulp serve
 
 When you open the web part toolbox to add the web part to the canvas, you will see that there are now three web parts that users can choose from.
 
-![Web part toolbox showing the preconfigured version of the web part](../../../../images/preconfiguredentries-three-configurations-toolbox.png)
+![Web part toolbox showing the preconfigured version of the web part](../../../../images/preconfiguredentries-three-configurations-toolbox.png)