Fix incorrect query example in joining-queries.md

[aysetas]

Description

This pull request fixes an issue in the joining-queries.md documentation file, where the Query::nested() example demonstrated an incorrect usage of the query method.

Issue

The example incorrectly used:

->query(Query::match()->field('author.name')->field('Steven'))

Updated the example to:

->query(Query::match()->field('author.name')->query('Steven'))

[aysetas]

Hi @babenkoivan,
I have created a pull request. When you have a chance, could you kindly review it?

[babenkoivan]

Thank you for fixing the documentation!