docs(prefer-focus): fix correct examples for prefer focus (#85)

Co-authored-by: Ben Monro <[email protected]>

Author: Belco90

docs/rules/prefer-focus.md

@@ -35,8 +35,10 @@ expect(foo).not.toEqual(document.activeElement);
 Examples of **correct** code for this rule:
 
 ```js
+expect(input).not.toHaveFocus();
 expect(document.activeElement).not.toBeNull();
 
+expect(input).toHaveFocus();
 expect(document.activeElement).toBeNull();
 ```
 

src/__tests__/lib/rules/prefer-focus.js

@@ -9,6 +9,8 @@ import * as rule from "../../../rules/prefer-focus";
 const ruleTester = new RuleTester();
 ruleTester.run("prefer-focus", rule, {
   valid: [
+    `expect(input).not.toHaveFocus();`,
+    `expect(input).toHaveFocus();`,
     `expect(document.activeElement).toBeNull()`,
     `expect(document.activeElement).not.toBeNull()`,
   ],