Fix doc for extra_flags

We use `extra_flags` in the Releases.yaml, not `extra_args`.

Author: zakame

generate.pl

@@ -157,7 +157,7 @@ =head1 DESCRIPTION
 
 =over 4
 
-=item extra_args
+=item extra_flags
 
 Additional text to pass to C<Configure>.  At the moment, this is necessary for
 5.18.x so that it can get the C<-fwrapv> flag.