Updating exam practice

Author: rosecers

_toc.yml

@@ -54,6 +54,7 @@ parts:
         - file: problems/ps_4/problem_set_4
         - file: problems/ps_5/problem_set_5
         - file: problems/ps_6/problem_set_6
+        - file: problems/exam_2_practice
   - caption: Appendices
     chapters:
       - file: appendices/notation

lecture_files/Lecture23.md

@@ -41,7 +41,7 @@ Stable equilibrium
     $\delta \underline{S} = 0$ and $\delta^2 \underline{S} < 0$ with
     respect to any perturbation.
 
- Unstable
+Unstable
     any perturbation, large or small, will result in the system reaching a different equilibrium
     state than its initial state. Unstable states have
     $\delta^2 \underline{S} > 0$. Note that it is possible for