Removes duplicate 'before' opening tag in MFTF CSS selectors example

[sethobey]

Description

Removes duplicate opening tag in extracting CSS selectors example of MFTF test prep docs.

Fixed Issues (if relevant)

1. No related issue

Contribution checklist

• [X] Pull request has a meaningful description of its purpose
• [X] All commits are accompanied by meaningful commit messages
• [X] All new or changed code is covered with unit/verification tests (if applicable)
• [ ] All automated tests passed successfully (all builds on Travis CI are green)
• [X] Changes to Framework doesn't have backward incompatible changes for tests or have related Pull Request with fixes to tests

[okolesnyk]

Closing this PR as outdated. Please feel free to submit new PRs and Issues. Thanks