Fix some typos in README.md

[samuela]

Thanks for submitting a PR! We want to make contributing to Paseto as easy as possible. Please read these instructions carefully:

• [x] Explain the motivation for making this change.
• [x] Provide a test plan demonstrating that the code is solid.
• [x] Match the code formatting of the rest of the codebase.
• [x] Make sure to add tests to help keep code coverage up.

Motivation (required)

What existing problem does the pull request solve?

Test Plan (required)

A good test plan has the exact commands you ran and their output.

If you have added code that should be tested, add tests.

Next Steps

• Small pull requests are much easier to review and more likely to get merged. Make sure the PR does only one thing, otherwise please split it.
• Make sure all tests pass, we will run automated tests, but you can run it yourself by running cargo test.

[CLAassistant]

All committers have signed the CLA.

[samuela]

cc @drshrey

[Mythra]

The content of this change looks good to me. Thanks for fixing the small typos!