[Picker] Fix activator type (#11875)

<!--
  ☝️How to write a good PR title:
- Prefix it with [ComponentName] (if applicable), for example: [Button]
  - Start with a verb, for example: Add, Delete, Improve, Fix…
  - Give as much context as necessary and as little as possible
  - Open it as a draft if it’s a work in progress
-->

### WHY are these changes introduced?

Fixes #0000 <!-- link to issue if one exists -->

<!--
  Context about the problem that’s being addressed.
-->

### WHAT is this pull request doing?

<!--
  Summary of the changes committed.

Before / after screenshots are appreciated for UI changes. Make sure to
include alt text that describes the screenshot.

  Include a video if your changes include interactive content.

If you include an animated gif showing your change, wrapping it in a
details tag is recommended. Gifs usually autoplay, which can cause
accessibility issues for people reviewing your PR:

  <details>
    <summary>Summary of your gif(s)</summary>
    <img src="..." alt="Description of what the gif shows">
  </details>
-->

### How to 🎩

🖥 [Local development
instructions](https://github.com/Shopify/polaris/blob/main/README.md#install-dependencies-and-build-workspaces)
🗒 [General tophatting
guidelines](https://github.com/Shopify/polaris/blob/main/documentation/Tophatting.md)
📄 [Changelog
guidelines](https://github.com/Shopify/polaris/blob/main/.github/CONTRIBUTING.md#changelog)

### 🎩 checklist

- [ ] Tested a
[snapshot](https://github.com/Shopify/polaris/blob/main/documentation/Releasing.md#-snapshot-releases)
- [ ] Tested on
[mobile](https://github.com/Shopify/polaris/blob/main/documentation/Tophatting.md#cross-browser-testing)
- [ ] Tested on [multiple
browsers](https://help.shopify.com/en/manual/shopify-admin/supported-browsers)
- [ ] Tested for
[accessibility](https://github.com/Shopify/polaris/blob/main/documentation/Accessibility%20testing.md)
- [ ] Updated the component's `README.md` with documentation changes
- [ ] [Tophatted
documentation](https://github.com/Shopify/polaris/blob/main/documentation/Tophatting%20documentation.md)
changes in the style guide

polaris-react/src/components/Picker/components/Activator/Activator.tsx

@@ -24,6 +24,7 @@ export const Activator = forwardRef<HTMLButtonElement, ActivatorProps>(
         disabled={disabled}
         onClick={onClick}
         className={classNames(styles.Activator, disabled && styles.disabled)}
+        type="button"
       >
         <BlockStack as="span" gap="100">
           {label && (