perldelta: Entry for [perl #122655].

5.20 regression: '"X" !~ /[x]/i'

It looks like this patch, included in 5.20.1, did not get a perldelta
entry for that release.

(cherry picked from commit 6ff8f25)

Author: khwilliamson

pod/perldelta.pod

@@ -453,6 +453,13 @@ crash, and had done so since Perl 5.10.  (In some cases the crash did not
 start happening till 5.16.)  The crash has, of course, been fixed.
 [perl #123542]
 
+=item *
+
+A regression has been fixed that was introduced in v5.20.0 (fixed in
+v5.20.1 as well as here) in which a UTF-8 encoded regular expression
+pattern that contains a single ASCII lowercase letter does not match its
+uppercase counterpart. [perl #122655]
+
 =back
 
 =head1 Known Problems