fix: typo "we knows" -> "we know"

tex/diffgeo/forms.tex

@@ -588,7 +588,7 @@ \section{Digression: $\Lambda^k(V^\vee)$ versus $(\Lambda^k(V))^\vee$}
 To see how ``difficult'' the wedge product will be if we use the second notation, let $V = \RR^3$,
 $\alpha = dx \wedge dy \in \Lambda^2(V^\vee)$, and $\beta = dz \in \Lambda^1(V^\vee)$.
 
-Then, we knows:
+Then, we know:
 \begin{itemize}
 	\ii $\alpha(\ee_1 \wedge \ee_2) = 1$.
 	\ii $\beta(\ee_3) = 1$.