Fix a comment typo: tvb_bcd_dig_to_wmem_packet_str() returns a WMEM-allocated
string, not an EP-allocated one.

Change-Id: I3918ed66429c588d4503d98e10dc460b099e550e
Reviewed-on: https://code.wireshark.org/review/3795
Reviewed-by: Jeff Morriss <jeff.morriss.ws@gmail.com>