Minor ArrowHelper Change

As discussed in [this](http://stackoverflow.com/questions/16801888/why-does-arrowhelper-not-hit-the-target) stackoverflow question, the `ArrowHelper` direction is slightly off when the desired direction is approximately "straight up" -- or "straight down".

A simple constant change fixes the problem adequately, without causing numerical problems. The max error is now about 1/4 degrees.