Remove error description of move

(1) `x` can be used in main() after the call to spawn(). Because the variables follow normal move semantics, though the keyword `move` is used, and i32 implements `Copy`.
(2) I remove this sentence because the previous sentence gives the referrence to `move closures`, and more description of `move` may be redundant.

Author: tclfs

src/doc/book/concurrency.md

@@ -127,9 +127,7 @@ thread may outlive the scope of `x`, leading to a dangling pointer.
 
 To fix this, we use a `move` closure as mentioned in the error message. `move`
 closures are explained in depth [here](closures.html#move-closures); basically
-they move variables from their environment into themselves. This means that `x`
-is now owned by the closure, and cannot be used in `main()` after the call to
-`spawn()`.
+they move variables from their environment into themselves.
 
 ```rust
 use std::thread;