[ERROR] Wrong key in `MilvusClient.delete()` return value documentation

[minganx]

The documentation for MilvusClient.delete() states the return dict contains "delete_cnt", but the actual key returned by the SDK is "delete_count".

Please update the docs to reflect the correct key: delete_count.

Reference: https://milvus.io/api-reference/pymilvus/v2.6.x/MilvusClient/Vector/delete.md