release_managers_guide: clean up github issue regex

- change $2 to $1 in the replacement part; $2 no longer exists after
  commit febfc89
- only search for "gh" or "github" at word boundaries; don't replace
  "gh" in e.g. "high #5"
- change `BEGIN{undef $/};` to the `-g` command-line option
- remove useless /m flag on the regex (we're not using `^` or `$`)

(cherry picked from commit d526cc3)

Porting/release_managers_guide.pod

@@ -808,7 +808,7 @@ an entry has been added by F<corelist-perldelta.pl>.
 
 You should add pod links for GitHub issue references thusly:
 
- $ perl -p -i -e'BEGIN{undef $/}; s{(?:GH|github)\s*#(\d+)}{L<GH #$1|https://github.com/Perl/perl5/issues/$2>}img' pod/perldelta.pod
+ $ perl -gpi -e 's{\b(?:GH|github)\s*#(\d+)}{L<GH #$1|https://github.com/Perl/perl5/issues/$1>}ig' pod/perldelta.pod
 
 Re-read the perldelta to try to find any embarrassing typos and thinkos;
 remove any C<TODO> or C<XXX> flags; update the "Known Problems" section