prefer-includes: Drop duplicated line in docs (#2548)

Author: anna-liepina

docs/rules/prefer-includes.md

@@ -19,10 +19,6 @@ This rule is fixable, unless the search expression in `Array#some()` has side ef
 array.indexOf('foo') !== -1;
 ```
 
-```js
-array.indexOf('foo') !== -1;
-```
-
 ```js
 string.lastIndexOf('foo') !== -1;
 ```