Update openCypher-Exercises-Answer-Key.ipynb (#526)

* Update openCypher-Exercises-Answer-Key.ipynb

As mentioned earlier in the notebook, the `MERGE` clause does not support partial pattern matches.  When matching against patterns in `MERGE`, the matches are either the entire pattern, or the entire pattern is created.

Because the pattern is a partial match, it currently creates a new "person {first_name: 'Dave'}" node for each of the new "person" added to the graph. 

By first doing a MERGE for (d:person {first_name: 'Dave'), when the partial match happens, the whole path is again created, but this time linking to the already existing "Dave" rather than creating new ones each time.

* Update openCypher-Exercises-Answer-Key.ipynb

fix quotes

---------

Co-authored-by: Michael Chin <mchin188@yahoo.com>

Author: mverkruyse

src/graph_notebook/notebooks/06-Language-Tutorials/02-openCypher/openCypher-Exercises-Answer-Key.ipynb

@@ -373,7 +373,8 @@
     "WITH [{first_name: 'Taylor', last_name: 'Hall'},{first_name: 'Kelvin', last_name: 'Fernsby'},{first_name: 'Ian', last_name: 'Rochester'}] as followers\n",
     "\n",
     "UNWIND followers as f\n",
-    "MERGE (n:person {first_name: f.first_name, last_name: f.last_name})-[r:follower]->(:person {first_name: 'Dave'})\n",
+    "MERGE (d:person {first_name: 'Dave'})\n",
+    "MERGE (n:person {first_name: f.first_name, last_name: f.last_name})-[r:follower]->(d)\n",
     "ON CREATE\n",
     "    SET r.creation='Created'\n",
     "ON MATCH\n",