paper1: Fix erroneous arm length formula

[chordowl]

This pull request replaces the two instances of sin in the formula for the arm length with cos.

There must've been a mix-up while this was written up, as the code uses cos: https://github.com/raphlinus/spline-research/blob/1a0fd3df09db517726309e899f165dc225e466e3/curves.js#L219 Now, paper and code match.

(If one were to use sin as originally written, the arm length has the wrong sign most (if not all) the time. Even if one fixes that, the resulting curve behaves rather weirdly; I noticed this while trying to implement this curve for use with my pen plotter.)

Best, and thanks for the write-up, Lukas