[flang][docs] Correct time of Flang OpenMP call and add link to calendar
The time in flang/docs/GettingInvolved.md says 8:00 a.m. Pacific, but
the scheduled meeting occurs at 7:30 a.m. Pacific. A link to the LLVM
community calendar was also added to both this and the entry for the
main biweekly flang call since it is not clear when the calls actually
happen.