Added HttpClient.PatchAsync method (dotnet#5262)

Author: SommerEngineering

xml/System.Net.Http/HttpClient.xml

@@ -62,7 +62,9 @@
   
 8.  <xref:System.Net.Http.HttpClient.PutAsync%2A>  
   
-9. <xref:System.Net.Http.HttpClient.SendAsync%2A>  
+9.  <xref:System.Net.Http.HttpClient.SendAsync%2A>  
+
+10. <xref:System.Net.Http.HttpClient.PatchAsync%2A>  
   
  <xref:System.Net.Http.HttpClient> is intended to be instantiated once and re-used throughout the life of an application. Instantiating an HttpClient class for every request will exhaust the number of sockets available under heavy loads. This will result in SocketException errors. Below is an example using HttpClient correctly.