Fix capitalization in all {index} roles within headings according to style guide

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
 ```
@@ -79,7 +79,7 @@ Each row of $P$ can be regarded as a probability mass function over $n$ possible
 
 It is too not difficult to check [^pm] that if $P$ is a stochastic matrix, then so is the $k$-th power $P^k$ for all $k \in \mathbb N$.
 
-### {index}`markov chains <single: Markov chains>`
+### {index}`Markov chains <single: Markov chains>`
 
 ```{index} single: Finite Markov Chains
 ```

lectures/linear_algebra.md

@@ -1073,7 +1073,7 @@ the left-hand side is a *matrix norm* --- in this case, the so-called
 For example, for a square matrix $S$, the condition $\| S \| < 1$ means that $S$ is *contractive*, in the sense that it pulls all vectors towards the origin [^cfn].
 
 (la_neumann)=
-#### {index}`neumann's theorem <single: neumann's theorem>`
+#### {index}`Neumann's theorem <single: Neumann's theorem>`
 
 ```{index} single: Linear Algebra; Neumann's Theorem
 ```