docs(prefer-equality-matcher): fix !== example (#1699)

Author: qqii

docs/rules/prefer-equality-matcher.md

@@ -30,5 +30,5 @@ Examples of **correct** code for this rule:
 ```js
 expect(x).toBe(5);
 expect(name).not.toEqual('Carl');
-expect(myObj).toStrictEqual(thatObj);
+expect(myObj).not.toStrictEqual(thatObj);
 ```