Add quotes around incorrect command name

Earlier, I kept typing the wrong command name:

    $ mdk script dev.php
    Unknown Moodle command script...

And I kept trying different variations, not realising the problem was that I was calling `mdk script` rather than `mdk run`. I thought it was just the error message from `mdk script`. I think i'd have noticed the problem earlier  it'd be clearer to quote the text. I've done this before too e.g. git does this:

    $ git foo
    git: 'foo' is not a git command. See 'git --help'.
    
    Did you mean this?
    	log