improve the GitHub link regular expression

Also, move the "add pod links" part *before* the section
about checking POD.

(cherry picked from commit febfc89)

Author: book

Porting/release_managers_guide.pod

@@ -799,6 +799,10 @@ For a MAINT release use something like this instead:
 Ideally, also fill in a summary of the major changes to each module for which
 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
+
 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
 with any serious issues for which fixes are not going to happen now; and
@@ -814,10 +818,6 @@ formatting, e.g.
  $ ./perl -Ilib ext/Pod-Html/bin/pod2html pod/perldelta.pod > \
      ~/perldelta.html
 
-You should add pod links for GitHub issue references thusly:
-
- $ perl -p -i -e'BEGIN{undef $/}; s{((?:GH|github)\s*#)(\d+)}{L<$1$2|https://github.com/Perl/perl5/issues/$2>}img' pod/perldelta.pod
-
 If you make changes, be sure to commit them.
 
 =for checklist skip BLEAD-POINT MAINT RC