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/include/types/server.h b/include/types/server.h
index 4847def..1cabb83 100644
--- a/include/types/server.h
+++ b/include/types/server.h
@@ -43,7 +43,7 @@
 #include <types/checks.h>
 
 
-/* server states. Only SRV_ST_DOWN indicates a down server. */
+/* server states. Only SRV_ST_STOPPED indicates a down server. */
 enum srv_state {
 	SRV_ST_STOPPED = 0,              /* the server is down. Please keep set to zero. */
 	SRV_ST_STARTING,                 /* the server is warming up (up but throttled) */