DOC: fix a confusing typo on a regsub example

DOC: fix a confusing typo on a regsub example

Sébastien reported a confusing example in the doc about regsub when used
with quotes. Nested quotes are already not trivial to grasp, but when
typos are there and result in something valid, it's even worse. The closing
quote ought to have been inside the brackets. However haproxy will not make
any difference because the single quotes delimit a word and the delimited
word remains the same. Let's just not add yet another level of confusion.