docs/release-calendar: update 20.2
In !6574 (merged) I fixed the dates, but I didn't realise there was one too many
releases in the list, as -rc3
had already been released and -rc4
was
about to be.
-rc4
was since released, so the next 20.2 release is now -rc5
; let's
update the calendar to that.
/cc @dbaker