ReleaseNotes: update 'you may prefer' link to 3.8

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@263031 91177308-0d34-0410-b5e6-96231b3b80d8

docs/ReleaseNotes.rst

@@ -11,8 +11,8 @@ Written by the `LLVM Team <http://llvm.org/>`_
 .. warning::
 
    These are in-progress notes for the upcoming Clang 3.9 release. You may
-   prefer the `Clang 3.7 Release Notes
-   <http://llvm.org/releases/3.7.0/tools/clang/docs/ReleaseNotes.html>`_.
+   prefer the `Clang 3.8 Release Notes
+   <http://llvm.org/releases/3.8.0/tools/clang/docs/ReleaseNotes.html>`_.
 
 Introduction
 ============