Fix grammar and wording in README.md

[monokkai]

Corrected minor grammatical issues in the README.

Summary

Fixed minor grammar issue:

1. contributing bugfixes and improvements -> contributing bug fixes and improvements
2. You'll notice that we used an HTML-like syntax; we call it JSX. -> You'll notice that we used an HTML-like syntax, which we call JSX.

How did you test this change?

[josephsavona]

Thanks for your interest in contributing. We’re open to PRs that fix bugs, increase test coverage, or add features in line w our vision for the project. We don’t generally accept subjective stylistic changes like this.