Try to fix the render code block in Microsoft doc. (SharePoint#935)

See the code block about `_renderPlaceHolders` method is not rendering correctly.

Author: lijunle

docs/spfx/extensions/get-started/using-page-placeholder-with-extensions.md

@@ -123,8 +123,6 @@ In the following steps, you'll modify the Hello World Application Customizer to
 
 8. Create a new `_renderPlaceHolders` private method with the following code:
 
-
-
 	  ```ts
 	    private _renderPlaceHolders(): void {
 
@@ -192,7 +190,6 @@ In the following steps, you'll modify the Hello World Application Customizer to
 		}
 	      }
 	    }
-
 	  ```
 
 	  * Use `this.context.placeholderProvider.tryCreateContent` to get access to the placeholder.