Update libcxx/docs/ABIGuarantees.rst

Co-authored-by: Louis Dionne <[email protected]>

Author: huixie90

libcxx/docs/ABIGuarantees.rst

@@ -206,7 +206,7 @@ CityHash has the problem that it drops some bits on the floor. Fixing the implem
 resulting in an ABI break.
 
 ``_LIBCPP_ABI_ATOMIC_WAIT_NATIVE_BY_SIZE``
--------------------------------------------------
+------------------------------------------
 This flag changes the implementation of ``atomic::wait()`` and ``atomic::notify_one()/notify_all()`` to use the
 native atomic wait/notify operations on platforms that support them based on the size of the atomic type, instead
 of the type itself. This changes the behaviour of ``atomic::wait()`` and ``atomic::notify_one()/notify_all()`` in