Docs: Add an example for the spread operator to prefer-spread.md (#7802…

…) (#7804)

* Spread example to the docs for prefer-spread

* small whitespace mistake in prefer-spread.md

* removed excess whitespace per vitorbal's comment

* making whitespace more consistant

docs/rules/prefer-spread.md

@@ -30,9 +30,7 @@ Examples of **incorrect** code for this rule:
 /*eslint prefer-spread: "error"*/
 
 foo.apply(undefined, args);
-
 foo.apply(null, args);
-
 obj.foo.apply(obj, args);
 ```
 
@@ -41,6 +39,10 @@ Examples of **correct** code for this rule:
 ```js
 /*eslint prefer-spread: "error"*/
 
+// Using the spread operator
+foo(...args);
+obj.foo(...args);
+
 // The `this` binding is different.
 foo.apply(obj, args);
 obj.foo.apply(null, args);