C#: Fix typo

Author: hvitved

csharp/ql/src/API Abuse/FormatInvalid.qhelp

@@ -18,7 +18,7 @@ should be taken into account:
   </li>
   <li>
     Missing arguments will result in a <code>System.FormatException</code> exception
-    being thrown
+    being thrown.
   </li>
 </ol>