chore(python): simplify nox steps in CONTRIBUTING.rst (#51)

Source-Link: googleapis/synthtool@26558ba
Post-Processor: gcr.io/repo-automation-bots/owlbot-python:latest@sha256:99d90d097e4a4710cc8658ee0b5b963f4426d0e424819787c3ac1405c9a26719

.github/.OwlBot.lock.yaml

@@ -1,3 +1,3 @@
 docker:
   image: gcr.io/repo-automation-bots/owlbot-python:latest
-  digest: sha256:bfe1222de1e4abc82562f5d13874f822fe766ef9427144645a5d794b5f1269e9
+  digest: sha256:99d90d097e4a4710cc8658ee0b5b963f4426d0e424819787c3ac1405c9a26719

CONTRIBUTING.rst

@@ -68,14 +68,9 @@ Using ``nox``
 We use `nox <https://nox.readthedocs.io/en/latest/>`__ to instrument our tests.
 
 - To test your changes, run unit tests with ``nox``::
+    $ nox -s unit
 
-    $ nox -s unit-3.6
-    $ nox -s unit-3.7
-    $ nox -s unit-3.8
-    $ nox -s unit-3.9
-
-- Args to pytest can be passed through the nox command separated by a `--`. For
-  example, to run a single test::
+- To run a single unit test::
 
     $ nox -s unit-3.9 -- -k <name of test>
 
@@ -145,7 +140,7 @@ Running System Tests
 - To run system tests, you can execute::
 
    # Run all system tests
-   $ nox -s system-3.8
+   $ nox -s system
 
    # Run a single system test
    $ nox -s system-3.8 -- -k <name of test>