DOC: peers: fix the protocol tag name in the doc

DOC: peers: fix the protocol tag name in the doc

The peers protocol has been using "HAProxyS" as a binary tag sent on the
wire since day one in 1.5-dev3 with commit 2b920a1af ("[MAJOR] Add new
files src/peer.c, include/proto/peers.h and include/types/peers.h for
sync stick table management"), regardless, the doc says the protocol
identifier is "HaproxyS". It is likely this got fixed in the code before
merging and not in the doc.

This should be backported to any release as the doc is wrong.