fix link to sentry-testkit tests

Author: Mause

docs/platforms/javascript/common/best-practices/sentry-testkit.mdx

@@ -58,7 +58,7 @@ test("collect performance events", function () {
 });
 ```
 
-You may see more usage examples in the [testing section](https://github.com/zivl/sentry-testkit/tree/master/__tests__) of sentry-testkit repository as well.
+You may see more usage examples in the [testing section](https://github.com/zivl/sentry-testkit/tree/master/packages/sentry-testkit/__tests__) of sentry-testkit repository as well.
 
 ### Testkit API