coroutines doc: fix a RST critical

sylvestre · sylvestre · commit 4139f0dedefe · 2023-09-22T23:11:57.000+02:00
```
/build/source/llvm/docs/Coroutines.rst:1781: CRITICAL: Title level inconsistent:

'``coro.outside.frame``' Metadata
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
```
diff --git a/llvm/docs/Coroutines.rst b/llvm/docs/Coroutines.rst
@@ -1779,7 +1779,7 @@ Metadata
 ========
 
 '``coro.outside.frame``' Metadata
-^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+---------------------------------
 
 ``coro.outside.frame`` metadata may be attached to an alloca instruction to
 to signify that it shouldn't be promoted to the coroutine frame, useful for
