[CMAKE] Rename the man pages generation target

Summary:
This diff makes the manpages generation target names consistent with the
rpc generation target.

Depends on D7470.

Test Plan:

ninja doc-manpage-bitcoind

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7471