Fix grammar and article usage in documentation

- Fix verb agreement in lifecycle.rst: 'creates and send' -> 'creates and sends'
- Fix article usage: 'a HTTP' -> 'an HTTP' in quickstart.rst and errorhandling.rst

docs/lifecycle.rst

@@ -83,7 +83,7 @@ will return data that is translated into an HTTP response.
     to a view function, handle errors, etc.
 #.  Flask translates View function return into WSGI response data, passes it to WSGI
     server.
-#.  WSGI server creates and send an HTTP response.
+#.  WSGI server creates and sends an HTTP response.
 #.  Client receives the HTTP response.