Fix incorrect rladmin command in create-certificates.md (fixes #3737)

[harshh6305]

Fixes #3737

Problem

The "Install certificates" section instructed users to check certificate status using:

   rladmin status certificates

This command doesn't work — rladmin has no certificates subcommand, resulting in:

   ERROR: invalid token 'certificates'

Fix

Replaced the incorrect command with an openssl x509 command to verify the installed certificate's expiration dates, as suggested in the issue.

---

Note

Low Risk
Documentation-only change to install/verify steps; no product code or security behavior is modified.

Overview
Fixes incorrect Install certificates guidance that told readers to run rladmin status certificates, which fails because rladmin has no certificates subcommand.

The post-install step is retitled to Verify the installed certificate and now uses openssl x509 against /etc/opt/redislabs/<cert-name>_cert.pem to show subject, issuer, and validity dates. The follow-on rladmin info cluster step for certificate details is removed so verification is a single, working command.

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[kaitlynmichael]

A few changes and then we can merge. Thanks!

[kaitlynmichael]

Could you also delete this step and its code block (lines 606–610)? rladmin info cluster prints cluster policies and tunables — repl_diskless, slave_ha, login_lockout_threshold — and no certificate data at all, so it can't show certificate details. It came in with the same commit as rladmin status certificates, so it's the same bug. The openssl command above already covers what this step was reaching for via -subject -issuer -dates.

[harshh6305]

@kaitlynmichael Thanks for the review! I've made all the requested changes
Updated the command to include -subject -issuer with the correct cert path
Updated the heading to "Verify the installed certificate:"
Removed the invalid "View certificate details" step (rladmin info cluster)