Switch from tox to black-nb in linting section of contributing guide (a…

…ws#3527)

* Revise linter section to switch from tox to black-nb

* Update PR template from tox to black-nb

.github/PULL_REQUEST_TEMPLATE.md

@@ -11,6 +11,6 @@ _Put an `x` in the boxes that apply. You can also fill these out after creating
 - [ ] I have read the [CONTRIBUTING](https://github.com/aws/amazon-sagemaker-examples/blob/master/CONTRIBUTING.md) doc and adhered to the example notebook best practices
 - [ ] I have updated any necessary documentation, including [READMEs](https://github.com/aws/amazon-sagemaker-examples/blob/master/README.md)
 - [ ] I have tested my notebook(s) and ensured it runs end-to-end
-- [ ] I have linted my notebook(s) and code using `tox -e black-format,black-nb-format`
+- [ ] I have linted my notebook(s) and code using `black-nb -l 100 {path}/{notebook-name}.ipynb`
 
 By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

CONTRIBUTING.md

@@ -36,12 +36,13 @@ Before sending us a pull request, please ensure that:
 1. Clone your fork of the repository: `git clone https://github.com/<username>/amazon-sagemaker-examples` where `<username>` is your github username.
 
 
-### Run the Linters
+### Run the Linter
 
-1. Install tox using `pip install tox`
-1. cd into the amazon-sagemaker-examples folder: `cd amazon-sagemaker-examples` or `cd /environment/amazon-sagemaker-examples`
-1. Run the following tox command and verify that all linters pass: `tox -e black-check,black-nb-check`
-1. If the linters did not pass, run the following tox command to fix the issues: `tox -e black-format,black-nb-format`
+Apply Python code formatting to Jupyter notebook files using [black-nb](https://pypi.org/project/black-nb/).
+
+1. Install black-nb using `pip install black-nb`
+1. Run the following black-nb command on each of your ipynb notebook files and verify that the linter passes: `black-nb -l 100 {path}/{notebook-name}.ipynb`
+1. Some notebook features such as `%` bash commands or `%%` cell magic cause black-nb to fail. As long as you run the above command to format as much as possible, that is sufficient, even if the check fails
 
 
 ### Test Your Notebook End-to-End