novice/git/02-collab.md: Use <object> to embed img/github-collaboration.svg

This is the preferred method for embedding SVGs, and is recommended by
Mozilla, who also say [1]:

  An img element can be used theoretically, too. However this
  technique doesn't work in Firefox before 4.0.

I'm running Firefox 24.8.0, and basic SVGs included via <img> do work,
but <image> calls inside those SVGs are ignored [2].

[1]: https://developer.mozilla.org/en-US/docs/Web/SVG/Tutorial/Getting_Started#Basic_properties_of_SVG_files
[2]: https://developer.mozilla.org/en-US/docs/Web/SVG/SVG_as_an_Image#Restrictions