Fix aws_region platform list in Anthropic connection docs

[takayoshi-makabe]

The Anthropic connection documentation describes the aws_region extra as applying only to the bedrock platform, but the hook also passes it to the aws platform (Claude Platform on AWS) client — added during review of #69003 ("Pass aws_region to the AnthropicAWS (Claude Platform on AWS) client"). The hook docstring already lists both platforms; this updates connections.rst to match, so users configuring platform="aws" know the key applies to them.

related: #69003

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[boring-cyborg]

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