[docs] Fix category for Starnix RFC

The original category of "PosixCompat" is not listed in _areas.yaml,
which causes RFC-0082 to not show up in the list of accepted RFCs on
fuchsia.dev. Change the category to the otherwise-unused "Foreign ABI
Compatibility".

Change-Id: Iee18da1e742888586009dee8c7ab4df2e78b1aa2
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/693242
Fuchsia-Auto-Submit: Alex Konradi <akonradi@google.com>
Reviewed-by: Kevin Lindkvist <lindkvist@google.com>
Commit-Queue: Alex Konradi <akonradi@google.com>