[libc++][doc] Removes a colon in a title.

Author: mordante

libcxx/docs/UsingLibcxx.rst

@@ -298,8 +298,8 @@ C++17 Specific Configuration Macros
   This macro is used to re-enable `set_unexpected`, `get_unexpected`, and
   `unexpected`.
 
-C++20 Specific Configuration Macros:
-------------------------------------
+C++20 Specific Configuration Macros
+-----------------------------------
 **_LIBCPP_DISABLE_NODISCARD_AFTER_CXX17**:
   This macro can be used to disable diagnostics emitted from functions marked
   ``[[nodiscard]]`` in dialects after C++17.  See :ref:`Extended Applications of [[nodiscard]] <nodiscard extension>`