message: update macOS build instructions
diff: @@ -83,7 +83,7 @@ First install [Homebrew](https://brew.sh/). Then you can run these commands in T
  brew install armmbed/formulae/arm-none-eabi-gcc
  brew cask install osxfuse && brew install tup
  brew install openocd
  -pip install PyYAML Jinja2 jsonschema
  +pip3 install PyYAML Jinja2 jsonschema
  ```
  #### Windows
  