doc: change broken fg(1) links to fg(1p)

The fg(1) links in the readline docs have moved
from `http://man7.org/linux/man-pages/man1/fg.1.html`
to `http://man7.org/linux/man-pages/man1/fg.1p.html`.
It also modifies the regex for replacing man page links
in docs by allowing optional character after number.
eg: fg(1) and fg(1p) will both be now parsed and replaced.

Fixes: https://github.com/nodejs/node/issues/11492
PR-URL: https://github.com/nodejs/node/pull/11504


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>