Update async-data.mdx

[farkerhaiku]

:sparkles: What kind of change does this PR introduce? (Bug fix, feature, docs update...)

Link to main rather than master (master is behind by a few dozen commits). Speaking of which, you might consider changing the github template checklist to refer to main rather than master.

:arrow_heading_down: What is the current behavior?

a link to an outdated branch

:new: What is the new behavior (if this is a feature change)?

a link to the current branch

:boom: Does this PR introduce a breaking change?

nope

:bug: Recommendations for testing

click the link in the docs

:memo: Links to relevant issues/docs

https://gorouter.dev/async-data

:thinking: Checklist before submitting

• [ ] I made sure all projects build.
• [ ] I updated CHANGELOG.md to add a description of the change.
• [x] I updated the relevant documentation.
• [ ] I updated the relevant code example.
• [ ] I rebased onto current master.

[farkerhaiku]

I'm not sure that it's necessary to do all of the other things in this checklist, since this is just a documentation update. If you would like me to jump through all those hoops I'll be happy to.

[csells]

good catch!