Fix change to font

[kprosise]

Changed to properly use roboto-flex.

QA: checked output in browser, used browser tools to verify font use.

This commit addresses issue FFTK-2123, "Update font"

PR Template and Checklist

Please complete as much as possible to speed up the reviewing process.

Readiness and adding reviewers as appropriate is required.

All PRs should be reviewed by a technical writer/documentation team and a peer.
If effecting customers—which is a majority of content changes—a member of Customer Success must also review.

Readiness

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• Merge after date or event
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Overview

Why merge this PR? What does it solve?

Checklist

• Run spelling and grammar check, preferably with linter.
• Avoid changing any header associated with a link/reference.
• Step through instructions (or ask someone to do so).
• Review for wordiness
• Match tone and style of page/section.
• Run make linkcheck.
• View HTML in a browser to check rendering.
• Use semantic newlines.
• follow best practices for commits.
  • Descriptive title written in the imperative.
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  • Mention any related issues numbers.
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Comments

Any thing else that a maintainer/reviewer should know.
This could include potential issues, rational for approach, etc.

[MiloCasagrande]

Looks good @kprosise