fix: typo in Python module name in module docstring

[william-davies]

I might be mistaken, but I think that the - has to be _ for the import to work. Here is my testing:

[I] ⋊> ~/my-beancount on mainline ⨯ python                                                                                          (beancount) 22:30:18
Python 3.11.5 (main, Sep 11 2023, 08:19:27) [Clang 14.0.6 ] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import importlib
>>> importlib.import_module('fava_plugins.split_income')
<module 'fava_plugins.split_income' from '/Users/williamdavies/opt/miniconda3/envs/beancount/lib/python3.11/site-packages/fava_plugins/split_income.py'>
>>> importlib.import_module('fava-plugins.split_income')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/williamdavies/opt/miniconda3/envs/beancount/lib/python3.11/importlib/__init__.py", line 126, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<frozen importlib._bootstrap>", line 1204, in _gcd_import
  File "<frozen importlib._bootstrap>", line 1176, in _find_and_load
  File "<frozen importlib._bootstrap>", line 1126, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 241, in _call_with_frames_removed
  File "<frozen importlib._bootstrap>", line 1204, in _gcd_import
  File "<frozen importlib._bootstrap>", line 1176, in _find_and_load
  File "<frozen importlib._bootstrap>", line 1140, in _find_and_load_unlocked
ModuleNotFoundError: No module named 'fava-plugins'
>>> importlib.import_module('fava-plugins')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/williamdavies/opt/miniconda3/envs/beancount/lib/python3.11/importlib/__init__.py", line 126, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<frozen importlib._bootstrap>", line 1204, in _gcd_import
  File "<frozen importlib._bootstrap>", line 1176, in _find_and_load
  File "<frozen importlib._bootstrap>", line 1140, in _find_and_load_unlocked
ModuleNotFoundError: No module named 'fava-plugins'
>>> importlib.import_module('fava_plugins')
<module 'fava_plugins' from '/Users/williamdavies/opt/miniconda3/envs/beancount/lib/python3.11/site-packages/fava_plugins/__init__.py'>

Please do let me know if I'm wrong! Otherwise, updating the docstring will help others avoid the head-scratching I just did :)

[yagebu]

Thanks! :)