Update README.md collections() documentation

[scotteuser]

Thank you for the great library! We are implementing it within https://www.drupal.org/project/ai within our 'AI Search' sub-module. While working on this I spotted that the documentation currently says 'collections()' but that method does not exist on in the control resource: https://github.com/probots-io/pinecone-php/blob/master/src/Resources/ControlResource.php, so it should say 'collection()' instead. Thanks!