Update portal/posts/mystification.md

Co-authored-by: Brian Rose <[email protected]>

Author: jukent

portal/posts/mystification.md

@@ -58,7 +58,7 @@ Thanks to everyone who was involved in our sprint day (alphabetical):
     build:
         uses: ProjectPythia/cookbook-actions/.github/workflows/build-book.yaml@main
         with:
-        build_commandl: 'jupyter-book build .'
+        build_command: 'jupyter-book build .'
     ```
     Reach out if this is causing problems!