Update lectures/eigen_II.md

DiaPorntipa · HengchengZhang · web-flow · commit 2315821ba19c · 2023-06-15T13:43:59.000+10:00
Co-authored-by: zhc &lt;79777246+HengchengZhang@users.noreply.github.com&gt;
diff --git a/lectures/eigen_II.md b/lectures/eigen_II.md
@@ -411,7 +411,7 @@ Thus, the rate of convergence is governed by the modulus of the second largest e
 
 
 (la_neumann)=
-## The Neumann series lemma
+## The Neumann Series Lemma
 
 ```{index} single: Neumann's Lemma
 ```
