JFR: PrettyWriter uses incorrect year specifier

Found by SonarCloud, actually.

For this commit:
https://github.com/openjdk/jdk/commit/ee3576a48b700df3d8ad4bd447346d4102b20818

It says:

"Week Year ("YYYY") should not be used for date formatting

Few developers are aware of the difference between Y for "Week year" and y for Year when formatting and parsing a date with SimpleDateFormat or DateTimeFormatter. That's likely because for most dates, Week year and Year are the same, so testing at any time other than the first or last week of the year will yield the same value for both y and Y. But in the last week of December and the first week of January, you may get unexpected results."

See also ErrorProne rule: https://errorprone.info/bugpattern/MisusedWeekYear