Update environment.yml

Author: DrDrij

environment.yml

@@ -8,7 +8,7 @@ dependencies:
   - pip:
     - jupyter-book==0.11.1
     - sphinx-multitoc-numbering==0.1.3
-    - quantecon-book-theme==0.2.3
+    - git+https://github.com/quantecon/quantecon-book-theme@rhs-toc-adjustments
     - sphinx-tojupyter==0.1.1
     - sphinxext-rediraffe==0.2.7
     - sphinx-exercise==0.1.1