Update milvus_client.py

[pavlobielousov]

Fixed return value documentation for the delete method

[sre-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: pavlobielousov
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[sre-ci-robot]

Welcome @pavlobielousov! It looks like this is your first PR to milvus-io/pymilvus 🎉

[XuanYang-cn]

Hi @pavlobielousov, thanks for your contribution! Could you please sign off your commits to pass the DCO check? You can do this by amending your commit with:

git commit --amend --signoff
git push --force

I'll review once the DCO is green. Thanks!