message: Fix the CI status badge in the README
diff: -Smithy Rust ![status](https://github.com/awslabs/smithy-rs/workflows/CI/badge.svg)
  +Smithy Rust [![CI on Branch `main`](https://github.com/awslabs/smithy-rs/actions/workflows/ci-main.yml/badge.svg)](https://github.com/awslabs/smithy-rs/actions/workflows/ci-main.yml)
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  Smithy code generators for Rust that generate clients, servers, and the entire AWS SDK.
  