Improve grammar in asyncio documentation (GH-2993)

mivade · Mariatta · commit 87c3c5de731a · 2017-08-04T00:20:42.000+10:00
"not only is it .." is the correct form, as opposed to:
"not only it is ..."
diff --git a/Doc/library/asyncio-dev.rst b/Doc/library/asyncio-dev.rst
@@ -52,7 +52,7 @@ Cancellation
 ------------
 
 Cancellation of tasks is not common in classic programming. In asynchronous
-programming, not only it is something common, but you have to prepare your
+programming, not only is it something common, but you have to prepare your
 code to handle it.
 
 Futures and tasks can be cancelled explicitly with their :meth:`Future.cancel`
