[clang][doc] Uses C++20 instead of C++2a.

Author: mordante

clang/docs/UsersManual.rst

@@ -4287,8 +4287,8 @@ Execute ``clang-cl /?`` to see a list of supported options:
       /Yc<filename>           Generate a pch file for all code up to and including <filename>
       /Yu<filename>           Load a pch file and use it instead of all code up to and including <filename>
       /Z7                     Enable CodeView debug information in object files
-      /Zc:char8_t             Enable C++2a char8_t type
-      /Zc:char8_t-            Disable C++2a char8_t type
+      /Zc:char8_t             Enable C++20 char8_t type
+      /Zc:char8_t-            Disable C++20 char8_t type
       /Zc:dllexportInlines-   Don't dllexport/dllimport inline member functions of dllexport/import classes
       /Zc:dllexportInlines    dllexport/dllimport inline member functions of dllexport/import classes (default)
       /Zc:sizedDealloc-       Disable C++14 sized global deallocation functions