fix snippet file path

Author: gewarren

xml/System/NullReferenceException.xml

@@ -114,7 +114,7 @@ A <xref:System.NullReferenceException> exception is thrown when you try to acces
 
      To address this problem, test the method's return value to ensure that it's not `null` before calling any of its members, as the following example does.
 
-     :::code language="csharp" source="~/snippets/csharp/System/NullReferenceException/Overview/nullreturn2a.cs" id="Snippet5":::
+     :::code language="csharp" source="~/snippets/csharp/System/NullReferenceException/Overview/nullreturn2.cs" id="Snippet5":::
      :::code language="fsharp" source="~/snippets/fsharp/System/NullReferenceException/Overview/nullreturn2a.fs" id="Snippet5":::
      :::code language="vb" source="~/snippets/visualbasic/VS_Snippets_CLR_System/system.nullreferenceexception.class/vb/nullreturn2a.vb" id="Snippet5":::