Fix indentation in _en/starter-kit/index.html

Two spaces should be used to indent HTML code, instead of tabs. [1]

[1]: https://google-styleguide.googlecode.com/svn/trunk/htmlcssguide.xml#Indentation
Tentatively fix issue #854

The images have their path hardcoded, which means they are broken
in the development site served by grunt (which has no web folder).
Merge pull request #907 from jacquerie/issue-854

Issue 854