[libcxx][doc] Use Phabricator links.

Use a link to the Phabricator review in the patch column.

Reviewed By: zoecarver, cjdb, #libc

Differential Revision: https://reviews.llvm.org/D100707

GitOrigin-RevId: 0c8fab9af7f996a5e346d0170b3ea091670517eb
1 file changed