githooks.txt: use correct "reference-transaction" hook name

The "reference transaction" hook was introduced in commit 6754159767
(refs: implement reference transaction hook, 2020-06-19). The name of
the hook is declared as "reference-transaction" in "refs.c" and
testcases, but the name declared in "githooks.txt" is different.

Signed-off-by: Bojun Chen <bojun.cbj@alibaba-inc.com>
Reviewed-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/githooks.txt

@@ -404,8 +404,8 @@ Both standard output and standard error output are forwarded to
 `git send-pack` on the other end, so you can simply `echo` messages
 for the user.
 
-ref-transaction
-~~~~~~~~~~~~~~~
+reference-transaction
+~~~~~~~~~~~~~~~~~~~~~
 
 This hook is invoked by any Git command that performs reference
 updates. It executes whenever a reference transaction is prepared,