`magit-process-yes-or-no-prompt-regexp': robustify regexp

It only needs to match ssh(1)'s "Are you sure you want to continue
connecting (yes/no)?" prompt for the time being, but it doesn't hurt
to make it a bit more receptive:  Make it also match an optional space
after the closing square bracket or paren, and maybe a colon instead
of a question mark at the end.

Signed-off-by: Pieter Praet <pieter@praet.org>