fix typing for mutate to allow for Object types...

It may be related to the problem with [this pr](#15).  Also see [here](https://discuss.dgraph.io/t/lambda-json-mutation/16143).  Hopefully this fixes the JSON mutation not working problem, either way this needs to be updated.

Author: jdgamble555

slash-graphql-lambda-types/types.d.ts

@@ -78,7 +78,7 @@ declare module "@slash-graphql/lambda-types" {
     graphql: (s: string, vars: Record<string, any> | undefined, ah?: AuthHeaderField) => Promise<GraphQLResponse>;
     dql: {
       query: (s: string, vars: Record<string, any> | undefined) => Promise<GraphQLResponse>;
-      mutate: (s: string) => Promise<GraphQLResponse>;
+      mutate: (s: string | Object) => Promise<GraphQLResponse>;
     };
     authHeader?: AuthHeaderField;
   };