docs: outdated Flow URL in pull request template

[wo-o29]

Summary

This PR updates the outdated Flow URL in the pull request template from flowtype.org to flow.org. The Flow static type checker's official website has moved from the old flowtype.org domain to flow.org, but the pull request template still references the old URL.

This change ensures that contributors are directed to the correct and current Flow documentation website when following the contribution guidelines.

How did you test this change?

I verified this change by:

1. Confirming the domain change: Verified that flowtype.org is the old domain and flow.org is the current official Flow website

flowtype.org	flow.org

2. Checking URL accessibility: Confirmed that flow.org is accessible and contains the current Flow documentation
3. Template validation: Ensured the markdown formatting and link structure remain intact after the URL update
4. Cross-reference check: Verified that other references to Flow in the codebase use the updated domain

The change is purely a URL update with no functional code changes, so no additional testing of React functionality was required.