add missing =back to Date::Parse POD

[aggrolite]

running perldoc on Date::Parse includes the following message in the output:

POD ERRORS
       Hey! The above document had some coding errors, which are explained below:

       Around line 325:
           You forgot a '=back' before '=head1'

This change suppresses the error.

[aggrolite]

closing due to duplicate patch: #4