Fix typo in _toc.yml

Author: rickecon

docs/book/_toc.yml

@@ -24,7 +24,7 @@ parts:
   chapters:
   - file: content/theory/derivations
   - file: content/intro/parameters
-  file: content/intro/variables
+  - file: content/intro/variables
 - caption: References
   chapters:
   - file: content/OGCore_references