initdb help message about WAL segment size

I propose the attached patch, aligning the help message with the docs. Any
reason not to?

(yes, I confused myself by trying --wal-segsize=1MB instead of just
--wal-segsize=1. I blame just stepping off an intercontinental flight and a
very early morning :P)

--
Magnus Hagander
Me: https://www.hagander.net/ <http://www.hagander.net/&gt;
Work: https://www.redpill-linpro.com/ <http://www.redpill-linpro.com/&gt;