docs: revise MyFirstContribution codelab for clarity and usability

This commit improves the MyFirstContribution codelab by performing a
comprehensive review focused on clarity, accessibility, and technical
accuracy.

Key improvements include:
- Corrected multiple grammatical, typographical, and syntactical errors.
- Refined the step-by-step instructions to be more intuitive for
  first-time contributors.
- Clarified Git terminology and removed ambiguous phrases.
- Improved formatting of code blocks, commands, and notes for better
  readability.
- Ensured alignment with the latest Git workflows and community
  contribution practices.

This revision makes the guide more welcoming and less error-prone,
especially for contributors with limited experience in Git or open-source
development. It lowers the barrier to entry by offering precise, polished
instructions that are easier to follow.

This effort supports the broader goal of improving documentation quality,
onboarding experience, and contributor retention across Git projects.

Signed-off-by: Arnav Kapoor <[email protected]>

Author: arnavk23