[libc++][doc] Fixes building the release notes.

Summary:
This libc++ 19 release notes are not longer the active notes and need to
be hidden to avoid build errors.

Test Plan: 

Reviewers: 

Subscribers: 

Tasks: 

Tags: 


Differential Revision: https://phabricator.intern.facebook.com/D60251571

Author: mordante

libcxx/docs/ReleaseNotes.rst

@@ -5,7 +5,7 @@
     :hidden:
 
     ReleaseNotes/18
-    ReleaseNotes/20
+    ReleaseNotes/19
 
 .. The release notes are in versioned files, but we make sure to keep publishing
 .. them in an unversioned ReleaseNotes.html page for external sites to reference.