Quick PR to fix typos in README

[taylorreiter]

PR checklist

• Describe the changes you've made.
• Describe any tests you have conducted to confirm that your changes behave as expected.
• If you've added new software dependencies, make sure that those dependencies are included in the appropriate conda environments.
• If you've added new functionality, make sure that the documentation is updated accordingly.
• If you encountered bugs or features that you won't address, but should be addressed eventually, create new issues for them.

@jasbneal caught some typos in the readme when looking through the peptigate outputs. This PR fixes those, as well as some other things that Grammarly highlighted.