Fix the Australian PG rating

As per https://www.classification.gov.au/classification-ratings/what-are-ratings

Fixes #11650
Well, sort of. I don't think it is possible to differentiate between them, as we'd be comparing the integer values, not the position in the list?

Emby.Server.Implementations/Localization/Ratings/au.csv

@@ -1,11 +1,11 @@
 Exempt,0
 G,0
 7+,7
+PG,15
 M,15
 MA,15
 MA15+,15
 MA 15+,15
-PG,16
 16+,16
 R,18
 R18+,18