Fixed README issue

Author: alexthomas93

README.md

@@ -195,6 +195,7 @@ Ensure that your vector index is created prior to executing this example.
 from neo4j import GraphDatabase
 from neo4j_graphrag.embeddings import OpenAIEmbeddings
 from neo4j_graphrag.indexes import upsert_vectors
+from neo4j_graphrag.types import EntityType
 
 NEO4J_URI = "neo4j://localhost:7687"
 NEO4J_USERNAME = "neo4j"
@@ -219,7 +220,7 @@ upsert_vectors(
     ids=["1234"],
     embedding_property="vectorProperty",
     embeddings=[vector],
-    entity_type="NODE"
+    entity_type=EntityType.NODE,
 )
 driver.close()
 ```