Minor change

Author: wchen-r7

HACKING

@@ -39,7 +39,7 @@ interactive session type specific for the type of exploit.
 3. Don't use "sleep". It has been known to cause issues with
 multi-threaded programs on various platforms running an older version of
 Ruby such as 1.8. Instead, we use "select(nil, nil, nil, <time>)" or
-Rex::sleep() throughout the framework. We have found this works around
+Rex.sleep() throughout the framework. We have found this works around
 the underlying issue.
 
 4. Always use Rex sockets, not ruby sockets.  This includes