Update Use-CSS-to-brand-pages.md (SharePoint#2978)

Modification to the CSSRegistration occurs in the masterfile, not the css file. The example reflects the change to the relevant masterpage file (since it referred to contoso, I made a general assumption contoso.masterpage exists)

Author: davidcfk

docs/solution-guidance/Use-CSS-to-brand-pages.md

@@ -54,7 +54,7 @@ When you view the file, you'll notice many comments in this format: `/* [Replace
 
 1. Make a copy of corev15.css and name it contosov15.css.
 
-2. Open contosov15.css and modify the CssRegistration line to point to contosov15.css from corev15.css, as shown in the following example.
+2. Open the relevant masterpage (in this example, contoso.masterpage) and modify the CssRegistration line to point to contosov15.css from corev15.css, as shown in the following example.
 
   ```XML
   <SharePoint:CssRegistration Name="Themable/contoso.css" runat="server" />
@@ -188,4 +188,4 @@ namespace AlternateCSSAppAutohostedWeb.Services
 ## See also
 
 - [SharePoint site branding and page customization solutions](sharepoint-site-branding-and-page-customization-solutions.md)    
-- [Branding and site provisioning solutions for SharePoint](branding-and-site-provisioning-solutions-for-sharepoint.md)
+- [Branding and site provisioning solutions for SharePoint](branding-and-site-provisioning-solutions-for-sharepoint.md)