doc: improper rendering of characters classes

[Akim Demaille <akim.demaille@…>]

I have not checked all the pages, but at least for Perl syntax, one can read

For example =[abc]=, will match any of the characters 'a', 'b', or 'c'.

with, visibly, `=' not being interpreted as expected. There is the same issue a couple of line below.

http://www.boost.org/doc/libs/1_52_0/libs/regex/doc/html/boost_regex/syntax/perl_syntax.html

Cheers.

[John Maddock]

(In [82672]) Fix quickbook markup. Fixes #7958.