Update README.md

[leonarddt05]

I made some changes, that are;

1. In the section Structure, the sentence: "This directory has tests for modules or classes that mock out any external dependencies." Typo: "mock" should be "mocks" for grammatical accuracy.

2. In the section Structure, the sentence: "pytest will print how what the users sees has changed," Typo: "how what" should be "what" for grammatical accuracy.

PR checklist:

• Purpose of the code is evident to future readers
• Tests included or PR comment includes a reproducible test plan
• Documentation is up-to-date
• A changelog entry was added to changelog.d for any user-facing change
• Change has no security implications (otherwise, ping security team)

If you're unsure about any of this, please see:

• Contribution guidelines!
• One of the more specific guides located here