fix YAML code block syntax highlighting

YAML strings containing % or @ characters are valid YAML 1.2 strings.
In YAML 1.1, they have to be enclosed in quotes to be treated
properly. Since rst uses the Python library Pygments with a YAML 1.1,
strings containing % or @ characters have to be updated tobe rendered
properly.
Merge pull request #41 from xabbuh/yaml-syntax-highlighting-fixes

fix YAML code block syntax highlighting