Fix `compute_decomposition` docs

[Jaybsoni]

Context: The example in the doc-string for qml.QFT.compute_decomposition() was incorrect.

[github-actions[bot]]

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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.70%. Comparing base (1070104) to head (713b1d8). Report is 2 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #6294 +/- ## ======================================= Coverage 99.70% 99.70% ======================================= Files 444 444 Lines 42236 42236 ======================================= Hits 42113 42113 Misses 123 123 ```

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