bug: fix default search_param in milvus.py

[hyunsir]

Describe the bug
A bug in haystack/blob/main/haystack/document_stores/milvus.py.
In MilvusDocumentStore, search_param, line 167:
self.search_param = search_param or {"nprobe": 10}
And in line 433:
param={"metric_type": self.metric_type, **self.search_param},
query with milvus2 will give an error due to this incorrect parameter passing

Error message
ERROR:pymilvus.decorators:RPC error: [search], <MilvusException: (code=1, message=fail to search on all shard leaders, err=All attempts results:
attempt #1: UnexpectedError, error: fail to Search, QueryNode ID=14, reason=Search 14 failed, reason [UnexpectedError] Error in virtual knowhere::DatasetPtr knowhere::IVF_NM::Query(const DatasetPtr&, const Config&, faiss::BitsetView) at IndexIVF_NM.cpp:216: [json.exception.out_of_range. 403] key 'nprobe' not found err %!w()
attempt #2 canceled
)>, <Time:{'RPC start': '2023-04-17 10:19:17.569321', 'RPC error': '2023-04-17 10:19:17.987552'}>

Error Reason
According to https://milvus.io/docs/search.md , search_params = {"metric_type": "L2", "params": {"nprobe": 10}, "offset": 5}
But in line 433, it will be search_params = {"metric_type": "L2", "nprobe": 10}, if user uses default self.search_param

Method of correction
change line 433 into:
param={"metric_type": self.metric_type, "params": self.search_param},
or change line167 and related describe:
self.search_param = search_param or {"params":{"nprobe": 10}}

[hyunsir]

#4681

[ZanSara]

Hey @hyunsir! I'm sorry but we don't take contributions for Milvus at this time. MilvusDocumentStore is deprecated and will be removed in 1.17. https://docs.haystack.deepset.ai/v1.16-unstable/reference/document-store-api#milvusdocumentstore. I will close this PR.