docs: label MAP@k notebook outputs consistently

[he-yufeng]

Description

Follow-up for #2309 after #2316 moved the metric calculation to map_at_k().

A few notebooks still displayed the resulting map_at_k() value as plain MAP or Mean average precision, which makes the output look like the non-@k metric even though the code is already computing MAP@k. This PR only updates those labels to MAP@K / MAP@k and fixes one broken markdown link in the same operationalization notebook paragraph.

Related Issues

Fixes #2309.

References

• Use map_at_k instead of map for ranking-metric reporting #2316

Checklist:

• I have followed the contribution guidelines and code style for this project.
• I have added tests covering my contributions.
• I have updated the documentation accordingly.
• I have signed the commits, e.g. git commit -s -m "your commit message".
• This PR is being made to staging branch AND NOT TO main branch.

Validation:

• Parsed the five changed notebooks as JSON.
• Ran git diff --check.
• Did not re-run the notebooks; the changes are stored output/source label updates only.

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