doc: a follow-up fix for commit 4d77cae.

Author: agentzh

doc/HttpLuaModule.wiki

@@ -5299,7 +5299,7 @@ Timeout for the reading operation is controlled by the [[#lua_socket_read_timeou
     sock:settimeout(1000)  -- one second timeout
     local data, err = sock:receive()
     if not data then
-        ngx.say("failed to read a packet: ", data)
+        ngx.say("failed to read a packet: ", err)
         return
     end
     ngx.say("successfully read a packet: ", data)