Lesson 13: Expression forced to be surrounded by spaces

[muzimuzhi]

In lesson 13, only runs-on: ${{ matrix.os }} passes the checks. Either runs-on: ${{matrix.os }} or runs-on: ${{ matrix.os}} fails.

But in lesson 11, there's no such restriction. if: ${{github.ref == 'refs/heads/master'}} passes the checks.

I understand surrounding expression by one space on each side is a good style to follow, but no spaces surrounded is valid expression syntax too (well it's not explicitly documented, but currently things like ${{matrix.os}} works for real workflow runs.