Revert "Update hw2.rmd, change the serial number"

This reverts commit 329d430dae81745736fcba7f3220d61bac2f4656.  The
commit makes a couple of ill-advised changes:

 * In cases where a slide has an identical setup to a previous slide,
   the commit changes the list item number from "1." to "2.".  Markdown
   is indifferent to this value, though, and numbers each list based on
   its current ordering (i.e., it renders a "1." in these cases
   regardless.)  Thus the change is pointless and only adds potential
   maintenance the source code should the slides be reordered.

 * More importantly, in other cases the list item number was removed for
   apparently aesthetic reasons.  Whatever they might be, their benefit
   is offset by breaking the functionality of the slides.  Slidify uses
   the list item as a cue to provide the input field and associated
   buttons.
Fix typo in HW2
Capitalize first letter of question

Sometimes the first letter of a question is capitalized and other
times it is not.  Make the former the convention since they seem to
always be full sentences.