Fix tape_expand.py documentation examples

[andrijapau]

Fixes some out-of-date doc string examples that were found.

Only the set_decomposition docstring is user facing, but it is good to keep internal doc strings up to date as well.

[github-actions]

Hello. You may have forgotten to update the changelog!
Please edit doc/releases/changelog-dev.md with:

• A one-to-two sentence description of the change. You may include a small working example for new features.
• A link back to this PR.
• Your name (or GitHub username) in the contributors section.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.60%. Comparing base (14c61a6) to head (866c49d).
Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6812   +/-   ##
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  Coverage   99.60%   99.60%           
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  Files         476      476           
  Lines       45215    45167   -48     
=======================================
- Hits        45035    44988   -47     
+ Misses        180      179    -1     

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[JerryChen97]

The modifications look good to me. However, shall we add some information in description to indicate that the changes made here would not be reflected to the rendered docs? since it seems that some of the methods involved here, e.g. create_decomp_expand_fn, have been totally invisible since the latest at the current point. (check via: https://pennylane.ai/search?contentType=DOC&q=create_decomp_expand_fn&version=latest)

It might be helpful as well if we can quickly check which user-faced API's call these functionalities here, and make sure no more adjustments are needed there. If it turns out to be a totally internal change which does not require further mods we might also want to mention in the PR description.