fix typos

content/Math4201/Math4201_L33.md

@@ -131,7 +131,7 @@ $$
 A=\{\underline{x}=(x_1,x_2,x_3,\dots)|x_i\in \{0,1\}\}
 $$
 
-Let $\underline{x} and \underline{x}'$ be two distinct elements of $A$.
+Let $\underline{x}$ and $\underline{x}'$ be two distinct elements of $A$.
 
 $$
 \rho(\underline{x},\underline{x}')=\sup_{i\in \mathbb{N}}\overline{d}(\underline{x}_\alpha,\underline{x}'_\alpha)=1