Fix typo in dynamical_systems.jmd

Missing $, so Latex formula isn't rendered

Author: ARKAD97

_weave/lecture04/dynamical_systems.jmd

@@ -77,7 +77,7 @@ The global behavior can then be categorized as:
 - If $| \alpha | < 1$, then $u_n \rightarrow 0$
 - If $| \alpha | > 1$, then $u_n \rightarrow \infty$
 
-If | \alpha | = 1$, then  complex dynamics can occur.
+If $| \alpha | = 1$, then  complex dynamics can occur.
 
 ## Nonlinear Geometric Dynamics