packet-http2.c: Fix a very minor bug

- incorrect fetch/display of hf_http2_push_promise_r field

Change-Id: Iddf2db4807713017519b5abcbff3ba824b3624c6
Reviewed-on: https://code.wireshark.org/review/4188
Reviewed-by: Bill Meier <wmeier@newsguy.com>