fix bad example

doc/apiv3.apib

@@ -317,7 +317,7 @@ already present in the core API.
             {
                 "_type": "Error",
                 "errorIdentifier": "urn:openproject-org:api:v3:errors:NotFound",
-                "message": "The specified version does not exist."
+                "message": "The specified work package does not exist."
             }
 
 ## Work package costs per type [/api/v3/work_packages/{id}/summarized_costs_by_type]
@@ -400,7 +400,7 @@ a `CostType` and the amount of `spentUnits`.
             {
                 "_type": "Error",
                 "errorIdentifier": "urn:openproject-org:api:v3:errors:NotFound",
-                "message": "The specified version does not exist."
+                "message": "The specified work package does not exist."
             }
 
 # Group Cost Types