Minor README fixes

[tomodachi94]

If there's anything else you think I missed or you don't want changed, don't hesitate to say so.

Changelog

README.md

• Markdown syntax fixes (commit c85e2b7292d28c5d76c47fef372f3fa4f5d1bea4)
  • Correct heading levels
  • Remove a random colon
• Correct links (commit bdce7af166eab08f578a6ab520db90881c00710c)
  • todotxt.com redirected to todotxt.org
  • The specification got moved to it's own repository on GitHub
• Minor fixes (commit b5e57fd26233d546ebea02b6eb0ce87593ef7fdb)
  • Convert some of the 'single quotes' to in-line code blocks (backticks)
  • Link to the "ToDoLib" folder
  • Correct typo ("I'm am happy" to "I am happy")