checkout doc: fix an unmatched double-quote pair

Signed-off-by: Yoichi Nakayama <yoichi.nakayama@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-checkout.txt

@@ -285,7 +285,7 @@ section of linkgit:git-add[1] to learn how to operate the `--patch` mode.
 +
 You can use the `"@{-N}"` syntax to refer to the N-th last
 branch/commit checked out using "git checkout" operation. You may
-also specify `-` which is synonymous to `"@{-1}`.
+also specify `-` which is synonymous to `"@{-1}"`.
 +
 As a special case, you may use `"A...B"` as a shortcut for the
 merge base of `A` and `B` if there is exactly one merge base. You can