strvec: use correct member name in comments

strvec: use correct member name in comments

In d70a9eb611 (strvec: rename struct fields, 2020-07-28), we renamed the
"argv" member to "v". In the same patch we also did the following rename
in strvec.c:

    -void strvec_pushv(struct strvec *array, const char **argv)
    +void strvec_pushv(struct strvec *array, const char **items)

and it appears that this s/argv/items operation was erroneously applied
to strvec.h.

Rename "items" to "v".

Signed-off-by: Linus Arver <redacted>
Signed-off-by: Junio C Hamano <redacted>