Update class_3.tex

Typo

class_3.tex

@@ -165,7 +165,7 @@ \chapter{Class 3}
 
 \Homework[Prove balanced binary sequences]{
 	Consider $S$ generated by the rules $X \implied 01X$ and $X \implied 10X$. 
-	Prove that, for all binary words $x$, we have that $x\ in S$ if and only iff every finite prefix of even length of $x$ has the same number of $0$s and $1$s.  
+	Prove that, for all binary words $x$, we have that $x\ in S$ if and only if every finite prefix of even length of $x$ has the same number of $0$s and $1$s.  
 }
 
 Hints.