[rllib] Fix doctest failure

[krfricke]

Why are these changes needed?

Lint was still failing (but only caught with doctest):

File "../../python/ray/rllib/utils/numpy.py", line ?, in default

Failed example:

    tree.traverse(make_action_immutable, d, top_down=False)

Exception raised:

    Traceback (most recent call last):

      File "/opt/miniconda/lib/python3.6/doctest.py", line 1330, in __run

        compileflags, 1), test.globs)

      File "<doctest default[4]>", line 1, in <module>

        tree.traverse(make_action_immutable, d, top_down=False)

    NameError: name 'make_action_immutable' is not defined

Related issue number

Checks

• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(