fix typo (dotnet#554)

Author: mairaw

xml/System.Net.Http/HttpMessageHandler.xml

@@ -35,7 +35,7 @@
  
 1. <xref:System.Net.Http.DelegatingHandler> - A class used to plug a handler into a handler chain. 
  
-2. <xref:System.Net.Http.HttpMessageHandler> - A simple to class to derive from that supports the most common requirements for most applications. 
+2. <xref:System.Net.Http.HttpMessageHandler> - A simple class to derive from that supports the most common requirements for most applications. 
  
 3. <xref:System.Net.Http.HttpClientHandler> - A class that operates at the bottom of the handler chain that actually handles the HTTP transport operations. 
  
@@ -203,4 +203,4 @@
  </Docs>
  </Member>
  </Members>
-</Type>
+</Type>