Correct the example given for a future, add punctuation where necessary
Add a small section on futures to the tutorial, remove use task::spawn as it is now included by default in prelude.rs
Fix typo
auto merge of #6537 : osaut/rust/master, r=bstrie

* The example given in future.rs was corrected.

* I have added a small section describing futures in tutorial on tasks. It is far from being complete as I am stil learning !