Fix syntax issues in deepbook-v3 doc.

[0xhipo]

Description

Describe the changes or additions included in this PR. Demo code in Deepbook doc is not working properly. Fix some syntax bugs.

Test plan

How did you test the new or updated feature?

No test included. Doc fix on code syntax issue.

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Release notes

Check each box that your changes affect. If none of the boxes relate to your changes, release notes aren't required.

For each box you select, include information after the relevant heading that describes the impact of your changes that a user might notice and any actions they must take to implement updates.

• [ ] Protocol:
• [ ] Nodes (Validators and Full nodes):
• [ ] Indexer:
• [ ] JSON-RPC:
• [ ] GraphQL:
• [ ] CLI:
• [ ] Rust SDK:
• [√] REST API: