release manager guide: cmpVERSION needs the previous release

The versions in release_managers_guide.pod are used as template
placeholders. cmpVERSION.pl should be run with the previous version, not
the version being prepared.

Author: haarg

Porting/release_managers_guide.pod

@@ -649,7 +649,7 @@ For each Perl release since the previous release of the current branch, check
 for modules that have identical version numbers but different contents by
 running:
 
- $ ./perl -Ilib Porting/cmpVERSION.pl --tag=v5.X.Y
+ $ ./perl -Ilib Porting/cmpVERSION.pl --tag=v5.LAST
 
 (This is done automatically by F<t/porting/cmp_version.t> for the previous
 release of the current branch, but not for any releases from other branches.)