Fix cast before we unsubscribe (#78776)

This was otherwise throwing an exception every shutdown.

Author: jasonmalinowski

src/VisualStudio/Core/Def/ProjectSystem/OpenTextBufferProvider.cs

@@ -340,7 +340,7 @@ public void Dispose()
             return;
         }
 
-        var runningDocumentTableForEvents = (IVsRunningDocumentTable)_runningDocumentTable;
+        var runningDocumentTableForEvents = (IVsRunningDocumentTable)_runningDocumentTable.Value;
         runningDocumentTableForEvents.UnadviseRunningDocTableEvents(_runningDocumentTableEventsCookie);
         _runningDocumentTableEventsCookie = 0;