DOC: fix code block not closed

This fixes an un-closed code block in the file usage docs, causing two code blocks (php & yml) to be joined.

docs/en/02_Developer_Guides/14_Files/07_File_Usage.md

@@ -74,6 +74,7 @@ class UsedOnTableExtension extends Extension
         $ancestorDataObjects[] = $grandParentIWantToLink;
     }
 }
+```
 
 ### Example YML file:
 
@@ -81,4 +82,4 @@ class UsedOnTableExtension extends Extension
 SilverStripe\Admin\Forms\UsedOnTable:
   extensions:
     - My\App\Extensions\UsedOnTableExtension
-```
+```