[libc++][NFC] Fix typos in the libc++ 21 release notes (#149536)

(cherry picked from commit d737fe2)

Author: ldionne

libcxx/docs/ReleaseNotes/21.rst

@@ -10,15 +10,15 @@ Written by the `Libc++ Team <https://libcxx.llvm.org>`_
 
 .. warning::
 
-   These are in-progress notes for the upcoming libc++ 20.0.0 release.
+   These are in-progress notes for the upcoming libc++ 21.0.0 release.
    Release notes for previous releases can be found on
    `the Download Page <https://releases.llvm.org/download.html>`_.
 
 Introduction
 ============
 
 This document contains the release notes for the libc++ C++ Standard Library,
-part of the LLVM Compiler Infrastructure, release 20.0.0. Here we describe the
+part of the LLVM Compiler Infrastructure, release 21.0.0. Here we describe the
 status of libc++ in some detail, including major improvements from the previous
 release and new feature work. For the general LLVM release notes, see `the LLVM
 documentation <https://llvm.org/docs/ReleaseNotes.html>`_. All LLVM releases may