defn functions inside let bindings are still available outside

I think this point is important, since in Javascript when you define a function inside a scope, this function is only available in this scope. Here ClojureScript allows a function defined in a `let` bindings (concept close to the `scope`) to be available outside the `let`. Not sure my change in the code nor this very comment is clear, but for a ClojureScript beginner like familiar with Javascript I felt this needed to be explained and explicit.