t3905: correct test title

We reference the non-existent option `git stash show --show-untracked`
when we really meant `--only-untracked`. Correct the test title
accordingly.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t3905-stash-include-untracked.sh

@@ -367,7 +367,7 @@ test_expect_success 'stash show --only-untracked only shows untracked files' '
 	test_cmp expect actual
 '
 
-test_expect_success 'stash show --no-include-untracked cancels --{include,show}-untracked' '
+test_expect_success 'stash show --no-include-untracked cancels --{include,only}-untracked' '
 	git reset --hard &&
 	git clean -xf &&
 	>untracked &&