fix references in diagrams

Author: dimitri-yatsenko

book/30-schema-design/060-diagrams.ipynb

@@ -10,7 +10,7 @@
     "\n",
     "Several diagramming notations have been used for designing relational schemas, as already introduced in the section on [Relational Theory](../20-concepts/01-relational.md). Common notations include:\n",
     "\n",
-    "* **Chen's Entity-Relationship (ER) Notation**: Uses rectangles for entities and diamonds for relationships. [http://doi.org/10.1201/9781439861776], [https://en.wikipedia.org/wiki/Entity%E2%80%93relationship_model], [@doi.org/10.1145/320434.320440]\n",
+    "* **Chen's Entity-Relationship (ER) Notation**: Uses rectangles for entities and diamonds for relationships. [@10.1201/9781003314455], https://en.wikipedia.org/wiki/Entity%E2%80%93relationship_model, [@doi.org/10.1145/320434.320440]\n",
     "* **Crow's Foot Notation**, **IE (Information Engineering) Notation**, [**Barker's Notation**](http://www.entitymodelling.org/): Uses symbols at line endpoints to indicate cardinality (one, many, optional) and does not distinguish between entity sets and relationship sets.\n",
     "* **UML Class Diagrams**: Adapted from object-oriented modeling.\n",
     "\n",