fix(index.d.ts): return more permissive type

fix #140

Author: Larry Botha

lib/index.d.ts

@@ -54,7 +54,7 @@ type FormState<Inf = Record<string, any>> = {
     isValidating: boolean;
     isModified: boolean;
   }>;
-  handleChange: () => void;
+  handleChange: () => any;
   handleSubmit: () => any;
 };