A couple small README fixes

[jasonrudolph]

The changes are easier to see using the rich diff:

• https://github.com/atom/xray/commit/78166c1a55dcae4566314409f99021b0c2288932?short_path=54baf06#diff-54baf06ad81bf47c433a8e6157ec46f6
• https://github.com/atom/xray/commit/06c871d13381117ef1abbf5e5fdfde5822202ba6?short_path=54baf06#diff-54baf06ad81bf47c433a8e6157ec46f6

I hope this is useful. 🤜🤛

[nathansobo]

Glad to have your eyes on our plans @jasonrudolph. Thanks for the polish.