doc/development: remove --lg-env option from pytest

Remove the `--lg-env` option from the pytest invocation in the
development documentation. The labgrid plugin isn't loaded automatically
when running pytest from the labgrid repository, so the option isn't
recognized and causes the tests to fail

Signed-off-by: David Lechner <dlechner@baylibre.com>

Author: dlech

doc/development.rst

@@ -36,7 +36,7 @@ Tests can now be run via:
 
 .. code-block:: bash
 
-   python -m pytest --lg-env <config>
+   python -m pytest
 
 Writing a Driver
 ----------------