fixing README.md file to update pip install command

[towfeeqfayaz11]

This is fix for issue: #621

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[alwayslove2013]

@towfeeqfayaz11 pip install vectordb-bench[pinecone] works in most cases. However, in some shells, the characters [ and ] are interpreted as wildcards, which can cause unexpected behavior. Adding quotes around it like pip install 'vectordb-bench[pinecone]', will prevent the shell from parsing those special characters and let pip handle them correctly instead.

Your change is really necessary and helpful. We truly appreciate your contribution to the project!