DOC: agent-check: fix typo in "fail" word expected reply

`tcpcheck_agent_expect_reply` expects "fail" not "failed"

This should fix github issue #876

This can be backported to all maintained versions (i.e >= 1.6) as of
today.

Signed-off-by: William Dauchy <w.dauchy@criteo.com>
(cherry picked from commit f8e795ca04a2e89b801c40843b1b79fd97251606)
Signed-off-by: Willy Tarreau <w@1wt.eu>
(cherry picked from commit df223005a3f61d19c140e8bffbb17ec8507cc486)
Signed-off-by: Willy Tarreau <w@1wt.eu>
(cherry picked from commit 91a40c4ba722d74b024bc6de5833fbca06ef3beb)
Signed-off-by: Willy Tarreau <w@1wt.eu>
1 file changed