Add example usage of the second parameter of the onSubmit method. (#1305

)

* Add example of second onSubmit method parameter.

More docs = more knowledge!

Closes: #1107

Related: #234

* Update docs/guides/typescript.md

LGTM

Co-Authored-By: beausmith <beau@beausmith.com>

docs/guides/typescript.md

@@ -14,7 +14,7 @@ similar to React Router 4's `<Route>`.
 
 ```typescript
 import * as React from 'react';
-import { Formik, FormikProps, Form, Field, FieldProps } from 'formik';
+import { Formik, FormikActions, FormikProps, Form, Field, FieldProps } from 'formik';
 
 interface MyFormValues {
   firstName: string;
@@ -26,7 +26,11 @@ export const MyApp: React.SFC<{}> = () => {
       <h1>My Example</h1>
       <Formik
         initialValues={{ firstName: '' }}
-        onSubmit={(values: MyFormValues) => alert(JSON.stringify(values))}
+        onSubmit={(values: MyFormValues, actions: FormikActions<MyFormValues>) => {
+            console.log({ values, actions });
+            alert(JSON.stringify(values, null, 2));
+            actions.setSubmitting(false)
+         }}
         render={(formikBag: FormikProps<MyFormValues>) => (
           <Form>
             <Field