Update RenderInterface::render() documentation to match Drupal coding standards

Problem/Motivation

Code examples in the documentation for \Drupal\Core\Render\RendererInterface::render() are using the old array declaration style of array('key' => 'value') rather than the short array declarations, ['key' => 'value'], as per the Drupal coding standard.

Steps to reproduce

View the code examples in the documentation documentation: https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Render%21...

Proposed resolution

Update the documentation.

Remaining tasks

None

User interface changes

None

API changes

None

Data model changes

None

Release notes snippet

TBD