fix dashes on online manual

[stdedos]

I am talking about −− https://xpra.org/manual

e.g. I cannot search --start-child on the online manual
On man xpra it works.

I've seen it "fixed" on many projects e.g. on Github; I just cannot remember the "exact" issue summary.

[Antoine Martin]

r21888 works for me. Hopefully this doesn't trigger warnings somewhere else... (I thought the escaping was there for a reason, maybe not)

Feel free to re-open if I've missed something.

[stdedos]

Manual search works, and some dashes checked do not come on the online manual. I'd say verified & complete.