Make headers in README.md consistent

[matt-development-work]

I realized that the headers I added for the directory descriptions in #38 are not consistent.

(e.g. the header for the "audio-basics" directory is "Audio Basics" and the header for the "decode-audio-data" is "decodeAudioData"). I did this because some directory names are associated with a specific method, whereas others are more of a description.

For consistency purposes, I think it makes sense for all headers to be capital letters separated by spaces. If there is a method name associated with the directory, then it will appear in the description.

Thoughts on this, @chrisdavidmills?

[chrisdavidmills]

@matt-eric sure, that makes sense to me. All I'd add is that MDN standard practice is for headings to use sentence case rather than all capitalized, so I'd prefer that.