Core.async's put! returns true if accepted

Hi! In the first example of the [core.async](https://funcool.github.io/clojurescript-unraveled/#channels) section the code shows the following:

```clojure
(put! ch 42)
;; Got a value: 42
;; => 42
```

> Note that put! returned the value that was just put to the channel.

Just running the example in the REPL the `put!` returns `true` instead of `42`.

Start with the first core.async example in the core.async section linked in the issue and verify its return value in a ClojureScript REPL. Update the example or accompanying note so it matches the observed put! result, then rerun the example to confirm the documentation is accurate.

Written by the indexing model from the issue text.