[docs] Fix icon name in the example

[amandeepmittal]

Why

Context Slack

How

This PR fixes the checkmark circle icon name in the code example in the Icons guide under Vector icons section.

Test Plan

By running docs locally.

Checklist

• Documentation is up to date to reflect these changes (eg: https://docs.expo.dev and README.md).
• Conforms with the Documentation Writing Style Guide
• This diff will work correctly for npx expo prebuild & EAS Build (eg: updated a module plugin).