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.

Author: darnuria

src/doc/tutorial.md

@@ -1750,6 +1750,24 @@ closures, but they also own them: that is, no other code can access
 them. Owned closures are used in concurrent code, particularly
 for spawning [tasks][tasks].
 
+Closures can be used to spawn tasks.
+A practical example of this pattern is found when using the `spawn` function,
+which starts a new task.
+
+~~~~
+use std::task::spawn;
+
+// proc is the closure which will be spawned.
+spawn(proc() {
+    debug!("I'm a new task")
+});
+~~~~
+
+> ***Note:*** If you want to see the output of `debug!` statements, you will need to turn on
+> `debug!` logging.  To enable `debug!` logging, set the RUST_LOG environment
+> variable to the name of your crate, which, for a file named `foo.rs`, will be
+> `foo` (e.g., with bash, `export RUST_LOG=foo`).
+
 ## Closure compatibility
 
 Rust closures have a convenient subtyping property: you can pass any kind of
@@ -1771,45 +1789,6 @@ call_twice(function);
 > in small ways. At the moment they can be unsound in some
 > scenarios, particularly with non-copyable types.
 
-## Do syntax
-
-The `do` expression makes it easier to call functions that take procedures
-as arguments.
-
-Consider this function that takes a procedure:
-
-~~~~
-fn call_it(op: proc(v: int)) {
-    op(10)
-}
-~~~~
-
-As a caller, if we use a closure to provide the final operator
-argument, we can write it in a way that has a pleasant, block-like
-structure.
-
-~~~~
-# fn call_it(op: proc(v: int)) { }
-call_it(proc(n) {
-    println!("{}", n);
-});
-~~~~
-
-A practical example of this pattern is found when using the `spawn` function,
-which starts a new task.
-
-~~~~
-use std::task::spawn;
-spawn(proc() {
-    debug!("I'm a new task")
-});
-~~~~
-
-If you want to see the output of `debug!` statements, you will need to turn on
-`debug!` logging.  To enable `debug!` logging, set the RUST_LOG environment
-variable to the name of your crate, which, for a file named `foo.rs`, will be
-`foo` (e.g., with bash, `export RUST_LOG=foo`).
-
 # Methods
 
 Methods are like functions except that they always begin with a special argument,