Fix an incorrect comment in JavaLibrary.bp.

The procedure for updating the nullability warnings file changed
during the review of https://r.android.com/801554 but this comment was
not updated to reflect that.

Test: n/a (only affects comments)
Fixes: 142060419
Change-Id: Iaa603cad40a5aac91931662dcefb665a447bd836
1 file changed