Removes duplicate tag in CSS selectors documentation example

[sethobey]

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

Description

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

Fixed Issues (if relevant)

1. N/A

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/verification tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)
• Changes to Framework doesn't have backward incompatible changes for tests or have related Pull Request with fixes to tests