Fix trivial mistake in error message.

[deven]

Motivation and Context

This commit fixes a bad error message.

Description

In read_profile_line(), when .strip_prefix('[') fails, the original error message was Profile definition must start with ], which is obviously wrong. This commit fixes the error message to be Profile definition must start with '[', which corrects the mistake in the original error message (fixing ] to be [), and adds the single quotes to match the formatting of the error message used when .strip_suffix(']') fails (Profile definition must end with ']').

Testing

No testing is needed here; this is solely a change to a hardcoded error message string.

Checklist

This is a trivial fix. Is it really interesting enough to mention in a changelog?

• For changes to the smithy-rs codegen or runtime crates, I have created a changelog entry Markdown file in the .changelog directory, specifying "client," "server," or both in the applies_to key.
• For changes to the AWS SDK, generated SDK code, or SDK runtime crates, I have created a changelog entry Markdown file in the .changelog directory, specifying "aws-sdk-rust" in the applies_to key.

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[landonxjames]

Running canary: https://github.com/smithy-lang/smithy-rs/actions/runs/12523787422

[landonxjames]

Canary and CI check succeeded, thanks for the contribution! Merging

[deven]

@landonxjames Looks like the merge stalled?

[landonxjames]

Oops, no it was late and I forgot to actually approve the change. Should go through now.