Corrected Typo (dotnet#402)

Author: JRAlexander

xml/System.Windows.Diagnostics/ResourceDictionaryDiagnostics.xml

@@ -183,7 +183,7 @@
         <ReturnType>System.EventHandler&lt;System.Windows.Diagnostics.StaticResourceResolvedEventArgs&gt;</ReturnType>
       </ReturnValue>
       <Docs>
-        <summary>Occurs when s static resource reference is resolved.</summary>
+        <summary>Occurs when a static resource reference is resolved.</summary>
         <remarks>
           <format type="text/markdown"><![CDATA[  
   
@@ -258,4 +258,4 @@
       </Docs>
     </Member>
   </Members>
-</Type>
+</Type>