Cleanup ConfigurationExtensions summary (#4281)

Remove garbage characters

Author: MSDN-WhiteKnight

xml/Microsoft.Extensions.Configuration/ConfigurationExtensions.xml

@@ -25,7 +25,7 @@
   </Base>
   <Interfaces />
   <Docs>
-    <summary>Extension methods for configuration classes./&gt;.</summary>
+    <summary>Extension methods for configuration classes.</summary>
     <remarks>To be added.</remarks>
   </Docs>
   <Members>