README: Fix typo (#27)

Summary:
Fix typo in the command to run unit test: s/unittests/unittest/.

Pull Request resolved: #27

Reviewed By: chantra

Differential Revision: D53873599

fbshipit-source-id: 98aa70678692b6bce05b38bf24f628d01fc0a334

README.md

@@ -37,7 +37,7 @@ python -m venv .venv
 poetry install
 
 # Test
-poetry run python -m unittests
+poetry run python -m unittest
 ```
 
 ## Running