README.md: Update install steps to escape brackets

[daverigby]

Problem

Some shells (e.g. zsh) treat the '[]' in the argument as a shell expansion, and fail to pass the correct argument to pip:

% pip3 install pinecone-client[grpc] 
zsh: no matches found: pinecone-client[grpc]

Solution

Fix by quoting the package name.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update
• Infrastructure change (CI configs, etc)
• Non-code change (docs, etc)
• None of the above: (explain here)

Test Plan

Confirmed install still works with zsh and bash on macOS.