message: fix typo and make wording tweaks
diff: # OSPRay using CMake.
  #
  # Once you have done a 'make install' of an existing OSPRay
  -# build, create a separate build directory and invoke CMake on this
  -# directory. If you have 'openosp_DIR' setup correctly to point to where
  -# you just installed OSPRay, then this should built the ospTutorial app
  -# from that install and NOT use your local build.
  +# build (will install to CMAKE_INSTALL_PREFIX), create a separate build
  +# directory and invoke CMake on this directory. If you have 'ospray_DIR'
  +# setup correctly to point to where you just installed OSPRay, then this
  +# should build the ospTutorial app from that install and NOT use your
  +# local build.
  cmake_minimum_required(VERSION 3.1)
  