Fix sp in Week3-derivativeSign.html
Another sp in differentiateUsingLimit.html

From upstream, it looks like.
productRuleTwoWays: Change first hint to actually be a hint

The first hint was simply the text "Let's do Method 1 first:". So you had
to click the hint button twice to see any hints. Very trivial, I know.
Merge pull request #66 from mateor/dev2

Week2: Fix some front-facing spell errors