fetch tests: correct a comment "remove it" -> "remove them"

Correct a comment referring to the removal of just the branch to also
refer to the tag. This should have been changed in my
ca3065e7e7 ("fetch tests: add a tag to be deleted to the pruning
tests", 2018-02-09) when the tag deletion was added, but I missed it
at the time.

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

t/t5510-fetch.sh

@@ -613,7 +613,7 @@ test_configured_prune_type () {
 			git rev-parse --verify refs/tags/newtag
 		) &&
 
-		# now remove it
+		# now remove them
 		git branch -d newbranch &&
 		git tag -d newtag &&