texi2html: Fix translation of section titles

texi2html: Fix translation of section titles

texi2html inserts a whitespace at the beginning and the end of a
section title, i.e. "<h1> Chapter blah </h1>", while makeinfo didn't
use a space "<h1>Chapter blah</h1>". Update the regexp that extracts
the section titles accordingly to allow spaces right after <h\d> and
before </h\1>...