Wrap the trait in a code block to avoid confusing javadoc

[sugmanue]

Background

Fixes the following warning:

> Task :smithy-model:javadoc
/Users/sugmanue/Sources/smithy/smithy-model/src/main/java/software/amazon/smithy/model/knowledge/PropertyBindingIndex.java:136: warning: unknown tag. Unregistered custom tag?
     * @nestedProperties trait.
       ^
/Users/sugmanue/Sources/smithy/smithy-model/src/main/java/software/amazon/smithy/model/knowledge/PropertyBindingIndex.java:151: warning: unknown tag. Unregistered custom tag?
     * @nestedProperties trait.
       ^
2 warnings

Testing

• How did you test these changes?

Links

• Links to additional context, if necessary
• Issue #, if applicable (see here for a list of keywords to use for linking issues)

---

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.