(manpages) update "HDD_LEAVE_SPACE_DEFAULT" regex to escape "."

[tianon]

Normally I would've combined this with #568 (caught them during the same reading), but I think this one is possibly slightly more controversial? 😅

This string in the documentation looks like a regular expression, which makes the unescaped . in it feel odd. I went for the "idiomatic Perl" style [.] escaping because I personally find it more readable, but I'm happy to swap to the more common \. or even just have the PR closed instead because I don't feel strongly about it at all. 😄

(I also double checked that the code wasn't actually using a regular expression that needed to be updated, and indeed confirmed that it has some code that does parsing and something like c == '.' for testing this case and the regex is purely documentation. 👍)