docs: fix 'persistant'/'peristent' typos in Redis Insight K8s install

[mrchatam]

Summary

Correct spelling in the Redis Insight Kubernetes install guide:

• heading: "persistant storage" → "persistent storage"
• body: "peristent volume" → "persistent volume"

Test plan

• Confirmed both corrected words are the intended spelling
• No other wording changes

---

Note

Low Risk
Documentation-only spelling corrections with no runtime or configuration impact.

Overview
Fixes spelling in the Redis Insight Kubernetes install guide (install-on-k8s.md): the section heading now says persistent storage (was "persistant"), and the intro under that section says persistent volume (was "peristent"). No other wording or behavior changes.

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[mrchatam]

Closing: Redis CLA is not signed for this account, so these docs PRs cannot proceed. Will reopen after CLA is signed if still needed.