Update developer_guide.md (#310)

Fix a 404

docs/developer_guide.md

@@ -50,4 +50,4 @@ Fix all errors before submitting a PR.
 
 To run specific tests, run `npm run test-nocov -t <regex>`, where `<regex>` is any pattern that matches the names of the test blocks that you want to run.
 
-See our recommendations for writing unit tests [here](https://github.com/lyft/amundsenfrontendlibrary/blob/master/docs/recommended-practices.md).
+See our recommendations for writing unit tests [here](https://github.com/lyft/amundsenfrontendlibrary/blob/master/docs/recommended_practices.md).