Adjust error message when bodies don't match in `aws-smithy-protocol-test` (#3736)

The left parameter is the expected one, see

<https://github.com/smithy-lang/smithy-rs/blob/dc1ffb8a0d2cf9ba413d7c62097797863b12db07/rust-runtime/aws-smithy-protocol-test/src/lib.rs#L389-L394>.

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