Fixed documentation

Author: squidfunk

docs/reference/diagrams.md

@@ -29,7 +29,7 @@ markdown_extensions:
       custom_fences:
         - name: mermaid
           class: mermaid-experimental
-          format: !!pymdownx.superfences.fence_code_format
+          format: !!python/name:pymdownx.superfences.fence_code_format
 ```
 
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