Lectures/*.md: Cleanup whitespace

I use two-space tabs, and I imagine others use 8-space tabs.  Markdown accepts
either 4-space or 1-tab indents for code [1], but mixing the two is probably
not a good idea.  I converted existing tabs to four spaces and stripped
stripped trailing whitespace with:

  $ sed -i 's/\t/    /g' Lectures/*.md
 $ sed -i 's/[[:space:]]*$//' *.md

[1]: http://daringfireball.net/projects/markdown/syntax#precode