DOC: config: Move req.hdrs and req.hdrs_bin in L7 samples fetches section

DOC: config: Move req.hdrs and req.hdrs_bin in L7 samples fetches section

req.hdrs and req.hdrs_bin are L7 sample fetches, not L6. They were in the wrong
section.

This patch may be backported as far as 1.8.

(cherry picked from commit 687a68e2d04900e84317d066de80891a7f848747)
Signed-off-by: William Lallemand <wlallemand@haproxy.org>
(cherry picked from commit 2616784a08c83f65b0e451bba8dcba5a844e8dac)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 5e3d0a0c0a4b3ca32dd11c82276750e880c4ed34)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>