changed example hook name it was specified wrongly in useful_hook doc.

[p-mishra1]

Thanks for your contribution and we appreciate it a lot. The following instructions would make your pull request more healthy and more easily get feedback. If you do not understand some items, don't worry, just make the pull request and seek help from maintainers.

Motivation

Updating the useful_hook doc example.

Modification

In the example of how to create a custom hook the hook name should be added to config instead a different name is given in the steps.

BC-breaking (Optional)

Use cases (Optional)

Checklist

1. Pre-commit or other linting tools are used to fix the potential lint issues.
2. The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
3. If the modification has potential influence on downstream projects, this PR should be tested with downstream projects, like MMDet or MMCls.
4. The documentation has been modified accordingly, like docstring or example tutorials.

[CLAassistant]

All committers have signed the CLA.

[OpenMMLab-Assistant-004]

Hi @p-mishra1 ！First of all, we want to express our gratitude for your significant PR in the mmdetection project. Your contribution is highly appreciated, and we are grateful for your efforts in helping improve this open-source project during your personal time. We believe that many developers will benefit from your PR.

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