Update stale --cpu-architecture options in the README.md

[oguzhan-yilmaz]

In the README.md, the following is stated about the CLI arguments:

-a, --cpu-architecture string                        CPU architecture [x86_64/amd64, x86_64_mac, i386, or arm64]

But running it with x86_64/amd64 fails.

ec2-instance-selector \
    --cpu-architecture "x86_64/amd64" \
    -o interactive

Error is:

NOTE: There was an error while parsing the commandline flags: error cpu-architecture must be one of: x86_64, x86_64_mac, amd64, i386, arm64

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