Fix workflow schema (#374)

Signed-off-by: Tihomir Surdilovic <[email protected]>

Author: tsurdilo

schema/workflow.json

@@ -361,7 +361,10 @@
                   "type": "object",
                   "description": "Function arguments/inputs"
                 },
-                "selectionSet": "Only used if function type is 'graphql'. A string containing a valid GraphQL selection set"
+                "selectionSet": {
+                  "type": "string",
+                  "description": "Only used if function type is 'graphql'. A string containing a valid GraphQL selection set"
+                }
               },
               "additionalProperties": false,
               "required": [