Porting/release_managers_guide.pod: use more neutral tag message

The tag message is just an example, but most releases aren't .0
versions, so "first release!" doesn't generalize well for release
engineers looking for guidance.

(Also, update the sample output to match the new tag example.)

Author: mauke

Porting/release_managers_guide.pod

@@ -973,7 +973,7 @@ Then delete the temporary installation.
 
 Create the I<annotated> tag identifying this release (e.g.):
 
- $ git tag v5.11.0 -m 'First release of the v5.11 series!'
+ $ git tag v5.21.4 -m 'Perl 5.21.4'
 
 It is B<VERY> important that from this point forward, you not push
 your git changes to the Perl master repository.  If anything goes
@@ -988,8 +988,9 @@ Verify that your tag is annotated:
 The output must look similar to the following:
 
  tag v5.X.Y
- Tagger: Jesse Vincent <[email protected]>
- Date:   Fri Oct 2 16:29:56 2009 -0400
+ Tagger: Steve Hay <[email protected]>
+ Date:   2014-09-20 11:09:51 +0100
+ ...
 
 =head3 Build the tarball