Changed the final URL to https://github.com/westes/flex

Signed-off-by: William Bailey <wbailey2@nd.edu>
(cherry picked from commit caf1d9292c)

HACKING

@@ -246,4 +246,4 @@ have done little to try to remove those warnings.
 If you do not have Flex installed, it can be downloaded from the
 following URL:
 
-   http://flex.sourceforge.net/
+    https://github.com/westes/flex