docs(model): Fix wrong description of "uuid" key in diagram cache index

This isn't actually a UUID (with the specific UUID format), but simply a
unique ID in the form of an opaque string.

In the interest of not breaking everything downstream, we'll keep the
key as "uuid" though.

capellambse/model/__init__.py

@@ -512,7 +512,7 @@ def update_diagram_cache(
             list of dictionaries, each representing a diagram in the
             model. The dictionaries come with the keys
 
-            - uuid: The UUID of the diagram
+            - uuid: The unique ID of the diagram
             - name: Name of the diagram as it has been set in Capella
             - type: The diagram type as it was created in Capella
             - viewpoint: The source layer from where the representation