update the label

there are two labels for this theorem.

Author: longye-tian

lectures/markov_chains_I.md

@@ -777,7 +777,6 @@ Sometimes the distribution $\psi_t = \psi_0 P^t$ of $X_t$ converges to $\psi^*$
 
 For example, we have the following result
 
-(strict_stationary)=
 ```{prf:theorem}
 :label: mc_gs_thm