fix: typo in UploadButton's docstring

[chaeheum3]

Description

Fixes the typo located in UploadButton class

Closes: #5545

[abidlabs]

Thanks @chaeheum3 for catching and fixing this

[gradio-pr-bot]

🪼 branch checks and previews

•	Name	Status	URL
🦄	Changes	detected!	Details

[gradio-pr-bot]

🦄 change detected

This Pull Request includes changes to the following packages.

Package	Version
gradio	patch

• Maintainers can select this checkbox to manually select packages to update.

With the following changelog entry.

fix: typo in UploadButton's docstring

Maintainers or the PR author can modify the PR title to modify this entry.

Something isn't right?

• Maintainers can change the version label to modify the version bump.
• If the bot has failed to detect any changes, or if this pull request needs to update multiple packages to different versions or requires a more comprehensive changelog entry, maintainers can update the changelog file directly.