Update Ruby MCP client example (#3212)

Follow-up to https://github.com/modelcontextprotocol/quickstart-resources/pull/177.

This PR removes the `base64` dependency from the `bundle add` arguments,
as it became a runtime dependency of Claude SDK for Ruby v1.60.0+.
https://github.com/anthropics/anthropic-sdk-ruby/pull/161#issuecomment-5221400980

At least when this draft of `build-client.mdx` was written, `bundle add ...` was expected to install
a recent version of the Claude SDK for Ruby that already includes `base64` as a runtime dependency.