add the reference back

HumphreyYang · HumphreyYang · commit 3ea0d374cded · 2023-06-01T10:31:59.000+10:00
diff --git a/lectures/markov_chains_II.md b/lectures/markov_chains_II.md
@@ -71,6 +71,7 @@ from matplotlib import cm
 import matplotlib as mpl
 ```
 
+(mc_irreducible)=
 ## Irreducibility
 
 
