Update build.yml

Author: knmcguire

.github/workflows/build.yml

@@ -11,5 +11,5 @@ jobs:
       - uses: actions/setup-python@v2
         with:
           python-version: 3.x
-      - run: pip install mkdocs pymdownx
+      - run: pip install mkdocs pymdown-extensions
       - run: mkdocs gh-deploy