updating changelog and readme to reflect openmp 4.0

README.md

@@ -74,7 +74,7 @@ Optional dependencies are:
   + TBB Device Adapter
       + [TBB](https://www.threadingbuildingblocks.org/)
   + OpenMP Device Adapter
-      + Requires a compiler that supports OpenMP >= 4.5.
+      + Requires a compiler that supports OpenMP >= 4.0.
   + OpenGL Rendering
       + The rendering module contains multiple rendering implementations
         including standalone rendering code. The rendering module also

docs/changelog/openmp-backend.md

 # OpenMP Device Adapter
 
-A device adapter that leverages OpenMP 4.5 for threading is now available. The
+A device adapter that leverages OpenMP 4.0 for threading is now available. The
 new adapter is enabled using the CMake option `VTKm_ENABLE_OPENMP` and its
 performance is comparable to the TBB device adapter.