Fix a typo ("is"->"if") in the comments; clarify a bit

flask/ctx.py

@@ -21,9 +21,9 @@ class _RequestGlobals(object):
 
 def has_request_context():
     """If you have code that wants to test if a request context is there or
-    not this function can be used.  For instance if you want to take advantage
-    of request information is it's available but fail silently if the request
-    object is unavailable.
+    not this function can be used.  For instance, if you want to take advantage
+    of request information if the request object is available, but fail
+    silently if it is unavailable.
 
     ::