Fix shell command example in README

The raw plugin config expects to receive objects. The README example for a shell command just passes the array of commands, making raw's check for [`target.command`](﻿https://github.com/docpad/docpad-plugin-raw/blob/master/src/raw.plugin.coffee#L36) impossible and producing the error `﻿﻿﻿﻿TypeError: Arguments to path.join must be strings`.