Edit _3gpp-nr-nrm-drachoptimizationfunction.yang

Updated revision date to conform to the rule in 32.160 clause 6.2.1.13

In order to avoid reusing the same revision date in multiple releases, when a new YANG module revision is needed the revision date should be set as follows. Instead of setting the exact revision date when the module was last edited, the date nearest to that day that is not in the future and that follows the rule below should be used.
When divided by 6, the day in the date should have the same remainder as the release number: (DAY modulo 6 == releaseNumber modulo 6).