fix(example): correct markup in the hello world example

I was asking this question myself. I figure that backticks are fine here since templates would be multi-line most of the time. But yes, in this precise example we could have single-quotes.

The other funny side-effect of using backticks is apparent here: we can't / shouldn't really use ES6 interpolation syntax (`${greeting}`) as it could "capture" local variables by accident...