fix: Fix small typo in Redis Pub/Sub docs

[soumitradev]

Description

Fix typo in code block in Redis Pub/Sub docs that misspelt the variable name of the duplicated Redis client.

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Checklist

• Does npm test pass with this change (including linting)?
• Is the new or changed code fully tested?
• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?

[leibale]

@soumitradev thanks!! :)