Refactor README.md formatting , correct typos and enhance grammar

[ismailbentabett]

Refactor README.md formatting, correct typos and enhance grammar

This pull request addresses various improvements to the README.md file to enhance its overall quality and readability. The changes encompass a comprehensive refactoring of formatting elements, including code snippets, headers, and bullet points, to create a visually consistent and appealing documentation structure.

Additionally, the pull request corrects several typographical errors present in the README.md, ensuring that the content is free from distracting mistakes and maintains a professional appearance.

Furthermore, there is a focus on refining the grammar throughout the document. This involves restructuring sentences for clarity, fixing grammatical inconsistencies, and ensuring a cohesive and polished narrative in the documentation.

These enhancements contribute to a more user-friendly and accessible README.md, providing users with a smoother onboarding experience and a better understanding of the Google Sign-In library for iOS.