now header is responsive

[Krishnapro]

📝 Description

I make header responsive for every screen by writing and updating media query in css.

🔧 Fixes #125

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📷 Provide related Screenshot/Gifs if any.

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• [x] I have performed a self-review of my own code.
• [x] I'm a contributor in Hacktoberfest'21.

[Krishnapro]

I make changes and create pull request please merge this @saloni-15