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

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 <redacted>
Reviewed-by: Patrick Steinhardt <redacted>
Signed-off-by: Junio C Hamano <redacted>