Add missing word

Doc/whatsnew/whatsnew25.tex

@@ -540,7 +540,7 @@ Traceback (most recent call last):
 StopIteration
 \end{verbatim}
 
-\keyword{yield} will usually return \constant{None}, you
+\keyword{yield} will usually return \constant{None}, so you
 should always check for this case.  Don't just use its value in
 expressions unless you're sure that the \method{send()} method
 will be the only method used to resume your generator function.