[futures.async] Use American English spelling of "behavior"

Fixes #463.

Author: jwakely

source/threads.tex

@@ -4695,7 +4695,7 @@
 
 \item
 If no value is set in the launch policy, or a value is set that is neither specified
-in this International Standard or by the implementation, the behaviour is undefined.
+in this International Standard or by the implementation, the behavior is undefined.
 \end{itemize}
 
 \pnum