[libc++][doc][NFC] update release notes on P3372R3 (#147161)

Author: huixie90

libcxx/docs/ReleaseNotes/21.rst

@@ -47,7 +47,7 @@ Implemented Papers
 - P1222R4: A Standard ``flat_set`` (`Github <https://github.com/llvm/llvm-project/issues/105193>`__)
 - P2897R7: ``aligned_accessor``: An mdspan accessor expressing pointer over-alignment (`Github <https://github.com/llvm/llvm-project/issues/118372>`__)
 - P3247R2: Deprecate the notion of trivial types (`Github <https://github.com/llvm/llvm-project/issues/118387>`__)
-- P3372R3: ``constexpr`` containers and adaptors (`Github <https://github.com/llvm/llvm-project/issues/128673>`__) (Only ``constexpr flat_map`` is implemented)
+- P3372R3: ``constexpr`` containers and adaptors (`Github <https://github.com/llvm/llvm-project/issues/127876>`__) (``forward_list``, ``list``, ``priority_queue``, ``flat_map``, and ``flat_set`` are implemented)
 - P2441R2: ``views::join_with`` (`Github <https://github.com/llvm/llvm-project/issues/105185>`__)
 - P2711R1: Making multi-param constructors of ``views`` ``explicit`` (`Github <https://github.com/llvm/llvm-project/issues/105252>`__)
 - P2770R0: Stashing stashing ``iterators`` for proper flattening (`Github <https://github.com/llvm/llvm-project/issues/105250>`__)