fix the variable name in the test comment for ${^WARNINGS_BITS}

[book]

This seems to have existed since 7e4f045 (the initial commit), so it definitely looks like a typo that wasn't spotted before.

• This set of changes does not require a perldelta entry.

[jkeenan]

Agreed.

$ ack -l '\$\{\^WARNING_HINTS\}' .
t/comp/hints.t

$ ack -l '\$\{\^WARNING_BITS\}' . |sort
cop.h
cpan/Pod-Simple/t/perlvaro.txt
cpan/Pod-Simple/t/perlvar.pod
cpan/Test-Simple/lib/Test2/EventFacet/Trace.pm
cpan/Test-Simple/lib/Test2/Util/Importer.pm
cpan/Test-Simple/lib/Test/Builder.pm
cpan/Test-Simple/lib/Test/More.pm
cpan/Test-Simple/t/modules/Bundle/Extended.t
cpan/Test-Simple/t/modules/V0.t
cpan/Text-Balanced/t/08_extvar.t
dist/Carp/lib/Carp.pm
ext/Pod-Html/corpus/perlvar-copy.pod
lib/B/Deparse.pm
lib/Carp.pm
lib/Test2/EventFacet/Trace.pm
lib/Test2/Util/Importer.pm
lib/Test/Builder.pm
lib/Test/More.pm
lib/warnings.pm
pod/perlfunc.pod
pod/perltoc.pod
pod/perlvar.pod
regen/warnings.pl
t/comp/hints.t
t/op/caller.t
t/re/reg_mesg.t