Update migration documentation with support for optional params

[pramodsum]

In the migration doc right now it states the following:

⚠️ You can't use regular expressions or optional params in v6 route paths. Instead, repeat the route with the extra params/regex patterns you're trying to match.

- <Route path="/one/:two?" component={Comp} />
+ <CompatRoute path="/one/:two" component={Comp} />
+ <CompatRoute path="/one" component={Comp} />

However, looks like it's been supported since v6.5.0. Can we update the docs to reflect this change 😊