Fix changelog parse failure (#4256)

## Motivation and Context
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## Description
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## Testing
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## Checklist
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- [ ] 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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Author: landonxjames

.changelog/hints-mostly-unused-lambda-rds.md

@@ -4,6 +4,8 @@ applies_to:
   - client
 authors:
   - joshtriplett
+references:
+  - smithy-rs#4208
 breaking: false
 new_feature: true
 bug_fix: false