RTD update: Change $echo PATH to which kpp in 01_installation.rst

docs/source/getting_started/01_installation.rst
- Now instruct users to type "which kpp" instead of "echo $PATH"
  (h/t Mike Long, @msl3v)

Signed-off-by: Bob Yantosca <yantosca@seas.harvard.edu>

docs/source/getting_started/01_installation.rst

@@ -60,10 +60,10 @@ Now if you type:
 
 .. code-block:: console
 
-   $ echo $PATH
+   $ which kpp
 
-You should see the :envvar:`$KPP_HOME/bin` directory placed at the end of
-the :envvar:`PATH` variable.
+the path to the executable file (:file:`kpp`) will be displayed. This
+path should match the path specified by :file:`$KPP_HOME/bin/kpp`.
 
 .. _test-for-dependencies: