t/README: link to metacpan.org, not search.cpan.org

Change a link to the web version of the TAP::Parser::Grammar
documentation to link to metacpan.org instead of search.cpan.org.

This is something I added back in commit 20873f45e7 ("t/README:
Document the do's and don'ts of tests", 2010-07-02), at the time
search.cpan.org was the more actively maintained CPAN web-interface,
nowadays that's metacpan.org.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/README

@@ -471,7 +471,7 @@ Don't:
    their output.
 
    You can glean some further possible issues from the TAP grammar
-   (see http://search.cpan.org/perldoc?TAP::Parser::Grammar#TAP_Grammar)
+   (see https://metacpan.org/pod/TAP::Parser::Grammar#TAP-GRAMMAR)
    but the best indication is to just run the tests with prove(1),
    it'll complain if anything is amiss.