Removed the explicit call to close and mentioned a bug that can occur

when two tasks is given the same id.

The explicit close call is not needed. We can do that in the `Drop`
implementation instead. It's better to have accounting tasks like this
only one place and it didn't add anything to the example.

There is a subtle bug which occurs if two tasks are given the same Id.
I mentioned this explicitly since it's such an easy thing to do. I
added the fix as a reader excercise since for the examples sake I think
it's better to pass them in explicitly so that we don't "pollute" the
example with more code than strictly needed to get an understanding of
futures.

README.md

@@ -4,6 +4,8 @@
 
 The rendered version is found at: [https://cfsamson.github.io/books-futures-explained/](https://cfsamson.github.io/books-futures-explained/)
 
+You can find the main example in the repository [examples-futures](https://github.com/cfsamson/examples-futures).
+
 This book aims to explain `Futures` in Rust using an example driven approach,
 exploring why they're designed the way they are, and how they work. We'll also
 take a look at some of the alternatives we have when dealing with concurrency