DOC: fix a few typos

include/types/proto_http.h: hwen -> when
include/types/server.h: SRV_ST_DOWN -> SRV_ST_STOPPED
src/backend.c: prefer-current-server -> prefer-last-server

Signed-off-by: Godbach <nylzhaowei@gmail.com>
diff --git a/src/backend.c b/src/backend.c
index e222160..70ddaa7 100644
--- a/src/backend.c
+++ b/src/backend.c
@@ -553,7 +553,7 @@
 	       (__objt_server(conn->target)->nbpend + 1) < s->be->max_ka_queue))) &&
 	    srv_is_usable(__objt_server(conn->target))) {
 		/* This session was relying on a server in a previous request
-		 * and the proxy has "option prefer-current-server" set, so
+		 * and the proxy has "option prefer-last-server" set, so
 		 * let's try to reuse the same server.
 		 */
 		srv = __objt_server(conn->target);