fix gen-manpages.sh to take into account shorter bitcoind --version line

Summary:
D7933 removed one word on the first line of the output of bitcoind --version.
The version number is now the 4th word on that line, and the manpage generation script should take this into account.

Depends on D7933

Test Plan:

$ src/bitcoind --version  | awk -F'[ -]' 'NR == 1 {print $4, $5}'
v0.22.5 66837b8c7

ninja doc-manpage-bitcoin-cli
cat doc/man/bitcoin-cli.1

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7946