Fix capitalization in index role for stochastic matrices heading

Co-authored-by: mmcky <8263752+mmcky@users.noreply.github.com>

Author: Copilot

lectures/finite_markov.md

@@ -64,7 +64,7 @@ from mpl_toolkits.mplot3d import Axes3D
 The following concepts are fundamental.
 
 (finite_dp_stoch_mat)=
-### {index}`stochastic matrices <single: stochastic matrices>`
+### {index}`Stochastic matrices <single: Stochastic matrices>`
 
 ```{index} single: Finite Markov Chains; Stochastic Matrices
 ```