Documentation updated for PITR
Documentation updated for PITR

The references for setting the time as part of a point-in-time-recovery
(PITR) in the documentation was incorrect. This moves it to the format
that pgBackRest actually uses.

Co-authored-by: Jonathan S. Katz <da75269b7a260628395a0deeca7fadc5e2e036a6@crunchydata.com>
CrunchyData/postgres-operator,CrunchyData/postgres-operator