Update using-asynchronous-methods-in-aspnet-45.md (dotnet#546)

Changed a 'has' to 'have', regarding grammar. No biggie, just doing my bit making this grammar impeccable :)

aspnet/web-forms/overview/performance-and-caching/using-asynchronous-methods-in-aspnet-45.md

@@ -43,7 +43,7 @@ This might not be a problem, because the thread pool can be made large enough to
 
 ## Processing Asynchronous Requests
 
-In web applications that see a large number of concurrent requests at start-up or has a bursty load (where concurrency increases suddenly), making web service calls asynchronous will increase the responsiveness of your application. An asynchronous request takes the same amount of time to process as a synchronous request. For example, if a request makes a web service call that requires two seconds to complete, the request takes two seconds whether it is performed synchronously or asynchronously. However, during an asynchronous call, a thread is not blocked from responding to other requests while it waits for the first request to complete. Therefore, asynchronous requests prevent request queuing and thread pool growth when there are many concurrent requests that invoke long-running operations.
+In web applications that see a large number of concurrent requests at start-up or have a bursty load (where concurrency increases suddenly), making web service calls asynchronous will increase the responsiveness of your application. An asynchronous request takes the same amount of time to process as a synchronous request. For example, if a request makes a web service call that requires two seconds to complete, the request takes two seconds whether it is performed synchronously or asynchronously. However, during an asynchronous call, a thread is not blocked from responding to other requests while it waits for the first request to complete. Therefore, asynchronous requests prevent request queuing and thread pool growth when there are many concurrent requests that invoke long-running operations.
 
 ## <a id="ChoosingSyncVasync"></a>  Choosing Synchronous or Asynchronous Methods