CLEANUP: mux-h2/trace: reformat the "received" messages for better alignment

user-level traces are more readable when visually aligned. This is easily
done by writing "rcvd" instead of "received" to align with "sent" :

  $ socat - /tmp/sock1 <<< "show events buf0"
  [00|h2|0|mux_h2.c:2465] rcvd H2 request  : [1] H2 REQ: GET /?s=10k HTTP/2.0
  [00|h2|0|mux_h2.c:4563] sent H2 response : [1] H2 RES: HTTP/1.1 200
1 file changed