Added $_regionCollection property to class

[maxalmonte14]

Description (*)

This PR fixes an error detected by PHPStan

Fixed Issues (if relevant)

1. https://github.com/magento-engcom/import-export-improvements/issues/136: Clean up Model/Import/Address.php

Manual testing scenarios (*)

1. Run ./vendor/bin/phpstan analyse app/code/Magento/CustomerImportExport/Model/Import/Address.php as described in https://github.com/magento-engcom/import-export-improvements/issues/136, after this implementation the following errors shouldn't be present

------ -------------------------------------------------------------------------------------------------------------------------------------------------------------------------
  Line   Address.php
 ------ -------------------------------------------------------------------------------------------------------------------------------------------------------------------------
  318    Access to an undefined property Magento\CustomerImportExport\Model\Import\Address::$_regionCollection.
  431    Access to an undefined property Magento\CustomerImportExport\Model\Import\Address::$_regionCollection.
 ------ -------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Contribution checklist (*)

• [x] Pull request has a meaningful description of its purpose
• [x] All commits are accompanied by meaningful commit messages
• [ ] All new or changed code is covered with unit/integration tests (if applicable)
• [ ] All automated tests passed successfully (all builds on Travis CI are green)

[magento-cicd2]

All committers have signed the CLA.

[maxalmonte14]

@dmanners thanks for the feedback, I'll apply the changes as soon as possible. ✌️

[maxalmonte14]

Updated, rookie mistake in the docblock 😅