fix(stokes): t should be theta

smh, see #247

tex/diffgeo/stokes.tex

@@ -386,7 +386,7 @@ \section{Boundaries}
 	[Boundary of a unit disk]
 	Consider the unit disk given by
 	\[ c \colon [0,1] \times [0,1] \to \RR^2 \quad\text{by}\quad
-	(r,\theta) \mapsto r\cos(2\pi t)\ee_1 + r\sin(2\pi t)\ee_2. \]
+	(r,\theta) \mapsto r\cos(2\pi \theta)\ee_1 + r\sin(2\pi \theta)\ee_2. \]
 	The four parts of the boundary are shown in the picture below:
 	\begin{center}
 		\begin{asy}