fix syntax highlighting example in Readme
fix syntax highlighting example in Readme

Markdown has to first output a <pre> with a "lang" attribute for syntax
highlighting filter to be able to pick it up.

nono/html-pipeline-linuxfr,kbrock/html-pipeline,charliesome/html-pipeline,terracatta/html-pipeline,kbrock/html-pipeline,moskvax/html-pipeline,github/html-pipeline,gjtorikian/html-pipeline,jch/html-pipeline,moskvax/html-pipeline,terracatta/html-pipeline,github/html-pipeline,gjtorikian/html-pipeline,tricknotes/html-pipeline,jch/html-pipeline