Rollup merge of rust-lang#63845 - DevQps:47091-remove-bad-example, r=nikomatsakis

Removed a confusing FnOnce example

# Description
See rust-lang#47091 for a discussion.

## Changes
- Removed an example that might suggest readers that square_x is (only) FnOnce.

closes rust-lang#47091

Author: Centril

src/libcore/ops/function.rs

@@ -185,14 +185,6 @@ pub trait FnMut<Args> : FnOnce<Args> {
 ///
 /// # Examples
 ///
-/// ## Calling a by-value closure
-///
-/// ```
-/// let x = 5;
-/// let square_x = move || x * x;
-/// assert_eq!(square_x(), 25);
-/// ```
-///
 /// ## Using a `FnOnce` parameter
 ///
 /// ```