Fix broken code example in FastField API docs + update CONTRIBUTING.m…

…d `master` references to `main` (#3958)

Title pretty much summarises it.

.github/CONTRIBUTING.md

@@ -69,8 +69,8 @@ git remote add upstream https://github.com/formik/formik.git
 3. Synchronize your local `next` branch with the upstream one:
 
 ```sh
-git checkout master
-git pull upstream master
+git checkout main
+git pull upstream main
 ```
 
 4. Install the dependencies with [yarn](https://yarnpkg.com) (npm isn't supported):
@@ -122,7 +122,7 @@ the results. If any of them fail, refer to [Checks and how to fix them](#checks-
 
 Make sure the following is true:
 
-- The branch is targeted at `master` for ongoing development. We do our best to keep `master` in good shape, with all tests passing. Code that lands in `master` must be compatible with the latest stable release. It may contain additional features, but no breaking changes. We should be able to release a new minor version from the tip of `master` at any time.
+- The branch is targeted at `main` for ongoing development. We do our best to keep `main` in good shape, with all tests passing. Code that lands in `main` must be compatible with the latest stable release. It may contain additional features, but no breaking changes. We should be able to release a new minor version from the tip of `main` at any time.
 - If a feature is being added:
   - If the result was already achievable with the library, explain why this feature needs to be added.
   - If this is a common use case, consider adding an example to the documentation.

docs/api/fastfield.md

@@ -57,7 +57,8 @@ const Basic = () => (
           alert(JSON.stringify(values, null, 2));
         }, 500);
       }}
-      render={formikProps => (
+    >
+      {formikProps => (
         <Form>
           {/** This <FastField> only updates for changes made to
            values.firstName, touched.firstName, errors.firstName */}
@@ -66,8 +67,8 @@ const Basic = () => (
 
           {/** Updates for all changes because it's from the
            top-level formikProps which get all updates */}
-          {form.touched.firstName && form.errors.firstName && (
-            <div>{form.errors.firstName}</div>
+          {formikProps.touched.firstName && formikProps.errors.firstName && (
+            <div>{formikProps.errors.firstName}</div>
           )}
 
           <label htmlFor="middleInitial">Middle Initial</label>
@@ -105,7 +106,7 @@ const Basic = () => (
            and all changes by all <Field>s and <FastField>s */}
           <label htmlFor="lastName">LastName</label>
           <Field name="lastName" placeholder="Baby">
-            {() => (
+            {({ field, form, meta }) => (
               <div>
                 <input {...field} />
                 {/**  Works because this is inside
@@ -125,7 +126,7 @@ const Basic = () => (
           <button type="submit">Submit</button>
         </Form>
       )}
-    />
+    </Formik>
   </div>
 );
 ```