Fix a few typos.

We should sit and chat for a bit about how you want the text to sound. E.g. if you want it to be less formal, the main page could say:

"Hello!"
Welcome to MightyScreen!
I am a tool for analyzing and displaying high-throughput screening data.
Let's get started."

Instead of my boring-but-more-descriptive description. I'm happy to help with the docs in the Fiber CMS also.