Use ‘an’ for $/=[] error message

This says ‘an ARRAY’:

$ perl -Mstrict -e '@{"a"}'
Can't use string ("a") as an ARRAY ref while "strict refs" in use at -e line 1.

This says ‘a ARRAY’:

$ ./miniperl -e '$/=[]'
Setting $/ to a ARRAY reference is forbidden at -e line 1.

It ought to say ‘an’.

1 files changed, 3 insertions, 1 deletions

diff --git a/mg.c b/mg.c
index 99a63f6844..c9bded4e0b 100644
--- a/mg.c
+++ b/mg.c
@@ -2769,7 +2769,9 @@ Perl_magic_set(pTHX_ SV *sv, MAGIC *mg)
                         );
                     }
                 } else {
-                    Perl_croak(aTHX_ "Setting $/ to a %s reference is forbidden", reftype);
+              /* diag_listed_as: Setting $/ to %s reference is forbidden */
+                    Perl_croak(aTHX_ "Setting $/ to a%s %s reference is forbidden",
+                                      *reftype == 'A' ? "n" : "", reftype);
                 }
             }
             SvREFCNT_dec(PL_rs);