[boost] regex docs match_flag_type.html

Just a minor thing I noticed in the regex++ docs: In match_flag_type.html it says that match_single_line is "Equivalent to Perl's s/ modifier". That is wrong, match_single_line is actually the inverse of Perl's /m modifier. Perl's /s causes "." to also match newlines, making it the inverse of match_not_dot_newline. - Karsten