Fix typo in HttpMessageHandler.xml (dotnet#5349)

The sentence:

> This method is called by the system one of the HttpClient.SendAsync methods is called. 

Should read:

> This method is called by the system when one of the HttpClient.SendAsync methods is called.

Author: ericmutta

xml/System.Net.Http/HttpMessageHandler.xml

@@ -245,7 +245,7 @@
 ## Remarks  
  This operation will not block. The returned <xref:System.Threading.Tasks.Task%601> object will complete once the entire response including content is read.  
   
- The <xref:System.Net.Http.HttpMessageHandler.SendAsync%2A> method is used primarily by the system. This method is called by the system one of the  <xref:System.Net.Http.HttpClient.SendAsync%2A?displayProperty=nameWithType> methods is called. Most apps will never call this method.  
+ The <xref:System.Net.Http.HttpMessageHandler.SendAsync%2A> method is used primarily by the system. This method is called by the system when one of the  <xref:System.Net.Http.HttpClient.SendAsync%2A?displayProperty=nameWithType> methods is called. Most apps will never call this method.  
   
  ]]></format>
         </remarks>