Fix application id for custom_callback example

[Wumpf]

rerun_example is a special string that causes us to flag recordings as example recordings on analytics (we don't send the appliation id to analytics ever)

[github-actions]

Web viewer built successfully. If applicable, you should also test it:

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Result	Commit	Link	Manifest
✅	f4771a1	https://rerun.io/viewer/pr/9135	+nightly +main

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