Fix incorrect function reference in comment of twophase.c

The header block of TwoPhaseGetDummyBackendId mentioned incorrectly
TwoPhaseGetDummyProc.

Reported-by: Oleksii Kliukin
Discussion: https://postgr.es/m/D8336E40-BBE1-4954-98BB-7830D3F5CB36@hintbits.com
