Minor fix in rocksample_problem.py

[zkytony]

Currently running python -m pomdp_py -r rocksample throws an exception

Traceback (most recent call last):
  File "/usr/local/lib/python3.10/runpy.py", line 196, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "/usr/local/lib/python3.10/runpy.py", line 86, in _run_code
    exec(code, run_globals)
  File "/home/kaiyu/pyenv/py310/lib/python3.10/site-packages/pomdp_py/__main__.py", line 31, in <module>
    main()
  File "/home/kaiyu/pyenv/py310/lib/python3.10/site-packages/pomdp_py/problems/rocksample/rocksample_problem.py",
 line 541, in main
    rocksample = debug_instance()  # create_instance(7, 8)
NameError: name 'debug_instance' is not defined. Did you mean: 'create_instance'?

This PR fixes that.