message: Call cmake on top level build tree rather than source.
diff: @@ -57,7 +57,7 @@ ADIOS2$ mkdir build && cd build
  3. Configure the project with CMake:
  ```bash
  -ADIOS2/build$ cmake -DCMAKE_INSTALL_PREFIX=/opt/adios2/2.3.1/gnu/openmpi ../source
  +ADIOS2/build$ cmake -DCMAKE_INSTALL_PREFIX=/opt/adios2/2.3.1/gnu/openmpi ../
  -- The C compiler identification is GNU 7.3.0
  -- The CXX compiler identification is GNU 7.3.0
  ...
  