04-howto.md: Assorted typo fixes and cleanups
04-howto.md: Use reference-style links to avoid URLs in the text

Besides making the text easier to read, reference-style links [1]
also:

* Isolate the text from changes to the URLs (e.g. no need to reflow if
  the lesson-manager repository moves to a new URL).
* Allows you to reuse a single URL if you wanted to link to Bower (or
  whatever) every time you mention it (instead of just the first
  time).
* Provides a convenient list of URLs at the end of the Markdown file
  which you can use to check for dead links later.

[1]: http://daringfireball.net/projects/markdown/syntax#link
Merge pull request #5 from wking/howto-typos

04-howto.md: Typo-fixes, reference-style links, and other cleanups