Don't try to link to the 4.0 release notes

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@294658 91177308-0d34-0410-b5e6-96231b3b80d8

docs/ReleaseNotes.rst

@@ -10,9 +10,9 @@ Written by the `LLVM Team <http://llvm.org/>`_
 
 .. warning::
 
-   These are in-progress notes for the upcoming Clang 5 release. You may
-   prefer the `Clang 4 Release Notes
-   <http://llvm.org/releases/4.0.0/tools/clang/docs/ReleaseNotes.html>`_.
+   These are in-progress notes for the upcoming Clang 5 release.
+   Release notes for previous releases can be found on
+   `the Download Page <http://releases.llvm.org/download.html>`_.
 
 Introduction
 ============