message: Add prolog syntax highlighting to md
diff: @@ -22,11 +22,11 @@ $ swipl load.pl
  ```
  The full suite of examples can be run with:
  -```
  +```prolog
  ?- run.
  ```
  or individual examples can be run with:
  -```
  +```prolog
  ?- runtest(Name). % e.g. runtest(sudoku).
  ```
  