fix(hooks): clarify that hooks is accessible through agent object

[dbschmigelski]

Description

Fix minor bug called out in #196 where hooks is not defined, since it should be called using the agent object.

Type of Change

• Typo/formatting fix

Motivation and Context

#196

Areas Affected

https://strandsagents.com/latest/documentation/docs/user-guide/concepts/agents/hooks/#registering-individual-hook-callbacks

Screenshots

Checklist

• I have read the CONTRIBUTING document
• My changes follow the project's documentation style
• I have tested the documentation locally using mkdocs serve
• Links in the documentation are valid and working
• Images/diagrams are properly sized and formatted
• All new and existing tests pass

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