Write the steps in lowercase.

This way, the filter script can replace them automatically with references to
the corresponding pages for each of the tutorial programs. This does not
currently work on http://dealii.org/developer/doxygen/deal.II/Tutorial.html .
Remove a duplicate section (thanks to Martin Kronbichler for pointing it out).
Merge pull request #55 from bangerth/tutorial-fixes

Write the steps in lowercase.