Minor correction in the documentation

[jistiak]

explicitly over-write all the num_classes field from default 15 to 5.

Here the num_classes should be 5, but it's given 15

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[zytx121]

LGTM

[openmmlab-bot]

Dear jistiak ,
First of all, we want to express our gratitude for your significant PR in the MMRotate 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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Best regards！ @jistiak