README.rst: change reference from "system" to "standard" modules

In the initial commit of this repo I included the README which gave an
example of the command line usage for the script. The example made
reference to a "--system" command line option. When I actually added the
script, this had been changed to "--standard" (as in "standard library")
but I forgot to change the README as well.
check_import_order: [fix] get abspath of py_file to get dirname

Without getting the abspath of the py_file before getting the dirname of
the it, when the user passes a file which is located in the working
directory, e.g. check_import_order.py test.py, then the `local_dir` arg
which is passed to `construct_import_dict` would be an empty string
which would be detected as "not a directory" by `os.path.isdir`, and thus
the script would fail and exit fatally.