Fix headline levesl in readme (#10)

Author: tcmetzger

README.md

@@ -74,7 +74,7 @@ is expected to follow the
 [Code of Conduct](https://github.com/bokeh/bokeh/blob/main/docs/CODE_OF_CONDUCT.md).
 This includes working on these tutorials!
 
-## Preparing your environment
+### Preparing your environment
 
 The ``bk-tutorial`` environment includes the necessary dependencies to contribute to
 this repository. The only exception if [pre-commit](https://pre-commit.com/), which
@@ -88,7 +88,7 @@ pre-commit install
 This way, some basic linting and formatting checks will be run on your code before
 you commit it.
 
-## Making changes
+### Making changes
 
 Contributing to this tutorial repository works similarly to
 [contributing to Bokeh itself](https://docs.bokeh.org/en/latest/docs/dev_guide.html):