fix: match input type with flow

Author: saihaj

src/type/schema.d.ts

@@ -69,7 +69,7 @@ export class GraphQLSchema {
 
   isSubType(
     abstractType: GraphQLAbstractType,
-    maybeSubType: GraphQLNamedType,
+    maybeSubType: GraphQLObjectType | GraphQLInterfaceType,
   ): boolean;
 
   getDirectives(): ReadonlyArray<GraphQLDirective>;