t5510: use the correct tag name in test

Fix an apparent copy-paste error: A few lines earlier, a tag
"refs/tags/sometag" is created.  Check for the (non-)existence of that
tag, not "somebranch", which is otherwise never mentioned in the
script.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5510-fetch.sh

@@ -121,7 +121,7 @@ test_expect_success 'fetch --prune --tags does not delete the remote-tracking br
 
 	git fetch --prune --tags origin &&
 	git rev-parse origin/master &&
-	test_must_fail git rev-parse somebranch
+	test_must_fail git rev-parse sometag
 '
 
 test_expect_success 'fetch --prune --tags with branch does not delete other remote-tracking branches' '