As we released 3.9, from the 4.0 release notes, points to version 3.9 instead of 3.8

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@286718 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 4.0.0 release. You may
-   prefer the `Clang 3.8 Release Notes
-   <http://llvm.org/releases/3.8.0/tools/clang/docs/ReleaseNotes.html>`_.
+   prefer the `Clang 3.9 Release Notes
+   <http://llvm.org/releases/3.9.0/tools/clang/docs/ReleaseNotes.html>`_.
 
 Introduction
 ============
@@ -121,7 +121,7 @@ OpenMP Support in Clang
 Internal API Changes
 --------------------
 
-These are major API changes that have happened since the 3.8 release of
+These are major API changes that have happened since the 3.9 release of
 Clang. If upgrading an external codebase that uses Clang as a library,
 this section should help get you past the largest hurdles of upgrading.