Update http.dart

Fixing dart-lang#27711

sdk/lib/io/http.dart

@@ -1668,13 +1668,13 @@ abstract class HttpClient {
                                            int port));
 
   /**
-   * Shutdown the HTTP client. If [force] is [:false:] (the default)
+   * Shut down the HTTP client. If [force] is [:false:] (the default)
    * the [:HttpClient:] will be kept alive until all active
    * connections are done. If [force] is [:true:] any active
    * connections will be closed to immediately release all
    * resources. These closed connections will receive an [:onError:]
-   * callback to indicate that the client was shutdown. In both cases
-   * trying to establish a new connection after calling [shutdown]
+   * callback to indicate that the client was shut down. In both cases
+   * trying to establish a new connection after calling [close]
    * will throw an exception.
    */
   void close({bool force: false});