update backdoor criterion

Author: 조해창

book/_toc.yml

@@ -7,7 +7,7 @@ parts:
     - file: prior_knowledge/overview.md
     - file: scm/overview.md
       sections:
-      - file: scm/graph_based_approach.ipynb
+      - file: scm/backdoor_criterion.ipynb
     - file: ate/overview.md
       sections:
       - file: ate/ols.ipynb