Made minor revisions and suggestions as to how some parts of the ractive...

....js tutorials could be clarified. Notes are for tutorials 1-9.
I felt as though it could be made clearer that format was a property, and that it's value was a function. You refer to "adding a function as 'data'" but then looking at the function alone (as on line 14 below) I still wasn't getting that this was meant to be the value for format. I think this is the first time a function is used as a property value, so I thought that showing the two together would be useful.