Edit copy for readability, convention

s/javascript/JavaScript and do some minor sentence wrangling in
Intro to Callbacks README
Make stylistic edits to copy and code

Across all files in intro to callbacks section:

- End JS statements w/ semicolon
- Use double quotes, not single quotes, by default. Singles reserved for
  apostrophes and cases where a single-quoted string is expressly needed
- Nitpicky grammar and spelling edits