docs: fix packaged app source link

[PuvaanRaaj]

Summary

Replace the deleted source revision linked from the application documentation with the current example source on master.

Fixes #1024

Validation

• git diff --check
• Verified examples/MyApp/src/MyApp.jl exists on upstream master through the GitHub Contents API
• Full Documenter build not run: Julia is not installed locally

AI assistance

Prepared with GPT-5.6 assistance and reviewed against the current upstream example.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.97%. Comparing base (7fa17bd) to head (7ed9c4c).

❗ There is a different number of reports uploaded between BASE (7fa17bd) and HEAD (7ed9c4c). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (7fa17bd)	HEAD (7ed9c4c)
	2	1

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1119      +/-   ##
==========================================
- Coverage   83.91%   73.97%   -9.94%     
==========================================
  Files           3        3              
  Lines         864      857       -7     
==========================================
- Hits          725      634      -91     
- Misses        139      223      +84     

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