Phrase review the main README.md file
Phrase review the main README.md file

* Updated several strings to increase clarity.
* Modified markdown formatting in a couple of places to make code examples easier to parse.
* Updated strings to active voice, if it didn't interfere with clarity or content.
gravitational/teleport,gravitational/teleport,gravitational/teleport,gravitational/teleport,gravitational/teleport,gravitational/teleport