Fix trivial mistake in error message. (#3952)

## 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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Co-authored-by: Landon James <lnj@amazon.com>