docs: fix example in prefer-screen-queries.md

Add missing import of "within" in example code.

Signed-off-by: Daniel Rentz <danielrentz@users.noreply.github.com>

docs/rules/prefer-screen-queries.md

@@ -38,7 +38,7 @@ getByText('foo');
 Examples of **correct** code for this rule:
 
 ```js
-import { render, screen } from '@testing-library/any-framework';
+import { render, screen, within } from '@testing-library/any-framework';
 
 // calling a query from the `screen` object
 render(<Component />);