📝 fix small

- [YAPF](https://github.com/google/yapf) (code format)

- [Flake8](https://flake8.pycqa.org/en/latest/) (code style check)

- [mypy](http://mypy-lang.org/) (type check)

- [Numpy Doctsring](https://numpydoc.readthedocs.io/en/latest/index.html) (API documents)

- [doctest](https://docs.python.org/3/library/doctest.html) (interactive examples)

Before making a PR, please check your codes using them.

You can confirm how to check your codes from [Coding Conventions](https://deepchem.readthedocs.io/en/latest/development_guide/coding.html).

## Document Conventions

DeepChem use [Sphinx](https://www.sphinx-doc.org/en/master/) to build [the document](https://deepchem.readthedocs.io/en/latest/index.html).

The document is automatically built by [Numpy style docstrings](https://numpydoc.readthedocs.io/en/latest/format.html#numpydoc-docstring-guide) in source codes and [Napoleon extension](http://www.sphinx-doc.org/en/stable/ext/napoleon.html).

It means you must add or modify Numpy style docstrings if you modify our codes.

It means you must add or modify Numpy style docstrings if you modify our codes or add new codes.

Before making a PR, please don't forget to modify not only codes but also docstrings.