docs(general): fix grammar in CONTRIBUTING.md

[yijiyap]

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

Description

Fixed sentence structure for creating a pull request, and example for environment set up in CONTRIBUTING.md

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my feature, policy, or fix is effective and works
• New and existing tests pass locally with my changes