DOC: hlua: fix a few typos in HTTPMessage.set_body_len() documentation

DOC: hlua: fix a few typos in HTTPMessage.set_body_len() documentation

A few typos were noticed while gathering info for the 3.2 announce
messages, this fixes them, and will probably constitute the last
commit of this release. There's no need to backport it unless commit
94055a5e7 ("MEDIUM: hlua: Add function to change the body length of
an HTTP Message") is backported.

(cherry picked from commit 21ce685fcdc2931cf6f5aafbf1f7cb68c9c7a1a1)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 240daee3aba09ceb508f5afecf2f7d26f0f06b06)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>