Fix pipenv install --dev help text

The previous help text made it seem that
`pipenv install --dev` would install ONLY
dev-packages, which is not the case.

The current description is identical to the
one used in the documentation, which is
a lot clearer.

pipenv/cli/options.py

@@ -166,7 +166,7 @@ def callback(ctx, param, value):
         state.installstate.dev = value
         return value
     return option("--dev", "-d", is_flag=True, default=False, type=click_booltype,
-                    help="Install package(s) in [dev-packages].", callback=callback,
+                    help="Install both develop and default packages.", callback=callback,
                     expose_value=False)(f)