Made failing/working examples look alike.

The failing concurrency example was doing something different from the
working example. This commit changes just enough of the failing example
to (1) still fail with the same error, (2) tries to do the same as the
working example (increment a vector value and print it).

r? @steveklabnik

src/doc/intro.md

@@ -428,7 +428,8 @@ fn main() {
 
     let guards: Vec<_> = (0..3).map(|i| {
         Thread::scoped(move || {
-            for j in 0..3 { numbers[j] += 1 }
+            numbers[i] += 1;
+            println!("numbers[{}] is {}", i, numbers[i]);
         });
     }).collect();
 }
@@ -437,10 +438,12 @@ fn main() {
 It gives us this error:
 
 ```text
-7:29: 9:10 error: cannot move out of captured outer variable in an `FnMut` closure
-7         Thread::scoped(move || {
-8             for j in 0..3 { numbers[j] += 1 }
-9         });
+7:25: 10:6 error: cannot move out of captured outer variable in an `FnMut` closure
+7     Thread::scoped(move || {
+8       numbers[i] += 1;
+9       println!("numbers[{}] is {}", i, numbers[i]);
+10     });
+error: aborting due to previous error
 ```
 
 It mentions that "captured outer variable in an `FnMut` closure".