fix : default num_partitions value from 16->64 in use-partition-key.md

[selfuppen]

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from source code of Milvus : https://github.com/milvus-io/milvus/blob/031ee6f155e5f508811492d4196e30946cb4aba0/internal/distributed/proxy/httpserver/handler_v2.go since this commit: https://github.com/milvus-io/milvus/commit/afa4193fc234cb964372760a151b71bd44ad071a the correct value is 64 in these versions (from v2.4.0 to now v2.4.9)

[sre-ci-robot]

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This pull-request has been approved by: AnthonyTsu1984, selfuppen

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