fix for dropdown toggle icon in cart

[Karlasa]

Description

Fixed cart page sidebar dropdown icon symbol.

Manual testing scenarios

1. Open cart page. Toggle discount or Tax dropdown.
   Expected: arrow down changes to arrow up.
   Currently: arrow down changes to arrow left.

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 @orlangur, thank you for the review.
ENGCOM-1941 has been created to process this Pull Request

[magento-engcom-team]

Hi @Karlasa. Thank you for your contribution.
Changes from your Pull Request will be available with the upcoming 2.2.6 release.

Please, consider to port this solution to 2.3 release line.
You may use Porting tool to port commits automatically.