[DOCS] Correct grammar on FrontendSetup.md

[thisdothassan]

Description

In the FrontendSetup.md, the second item under tech overview is "We using CSS (Bootstrap v5) for the styling" instead of "We are using CSS (Bootstrap v5) for the styling". There is a missing "are"

Screenshots

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🥦 Browser

Google Chrome

Checklist ✅

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• I have read the Contributing Guidelines
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• I am a GSSoC23 contributor (blank for no).