Removing '15.3 Do syntax' in tutorial.

The 'do' keyword was deprecated in 0.10 #11868 , and is keep as
reserved keyword #12157 .

So the tutorial part about it doesn't make sense.
The spawning explanation was move into '15.2 Closure compatibility'.

Fixing misspelling.

Thanks for precisions.

Moved from 15.2 to 15.1.

Fixed typo, and apply pnkfelix advices.
auto merge of #12367 : darnuria/rust/tutorial_removing_do_syntax, r=cmr

The 'do' keyword was deprecated in 0.10 #11868 , and is keep as
reserved keyword  #12157 .

So the tutorial part about it doesn't make sense.
The spawning explanation was move into '15.2 Closure compatibility'.