Spelling Correction (SharePoint#3801)

Updated word "fo" to "of"

Author: rdorrani

docs/spfx/extensions/guidance/using-custom-dialogs-with-spfx.md

@@ -60,7 +60,7 @@ To create a custom dialog box, you need to follow the steps in [Set up your deve
 
   ![SharePoint client-side solution scaffolded successfully](../../../images/ext-com-dialog-yeoman-complete.png)
 
-6. When initial scaffolding is completed, enter the following to explicitly install the 5.x version fo the Office UI Fabric to your solution:
+6. When initial scaffolding is completed, enter the following to explicitly install the 5.x version of the Office UI Fabric to your solution:
 
   ```sh
   npm install [email protected]  --save