[fix] typo in private method name getUniq[ue]ImageIndex

[mhauri]

Description

Private method name \Magento\Catalog\Console\Command\ImagesResizeCommand::getUniqImageIndex contained typo/misspelling.

Renamed it to getUniqueImageIndex.

Contribution checklist

• Pull request has a meaningful description of its purpose
• 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-engcom-team]

Hi @mhauri. Thank you for your contribution.
Changes from your Pull Request will be available with the upcoming 2.2.5 release.

Please, consider porting this solution across release lines.
You may use Porting tool to port commits automatically.