Write `typename' in templates rather than `class'.

Change-Id: I4d86f3985dc5161da221e0f29d9077b08d33461e
Reviewed-on: https://code-review.googlesource.com/c/re2/+/57830
Reviewed-by: Paul Wankadia <junyer@google.com>
(cherry picked from commit f8aee58c6db233f1cc8b3697fcf0f5fea02f2fa8)
Reviewed-on: https://code-review.googlesource.com/c/re2/+/57831