Update 'bbox_head' to align with new config files

[jistiak]

In the new version of the config files, the 'bbox_head' dict is inside 'roi_head'. If one follows the old documentation, it results in this following issue -

#607

This way 'bbox_head' is not repeated and the error is solved.

[zytx121]

Hi @jistiak,
Thanks for your kind PR. It seems that your branch is out of date and causes much difference.
Now, we require all the PRs to be merged into the development branch, i.e., dev branch for MMRotate 0.x and dev-1.x branch for MMRotate 1.x.
If your modification is not checkout from these two branches, they may cause conflicts and you should add your modification based on these two branches.
If your modification is checkout from these two branches, your branch might be out of date. In such a case
Please rebase your branch to the target dev or dev-1.x branch. You can simply do that by modifying your pull behavior:

git config --local --add pull.rebase true  # make rebase a default behavior in pull

Then you can git pull and rebase the dev branch by

git pull dev-1.x  # if your modification is for MMRotate 1.x
git pull dev  # if your modification is for MMRotate 0.x

After rebase, you might need to add --force option when pushing code, e.g.,

git push [remote name] [you branch] --force

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
2 out of 5 committers have signed the CLA.

✅ RangiLyu
✅ zytx121
❌ MinkiSong
❌ yangxue0827
❌ jistiak
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