(agent): Fix global.sysdig.accessKeySecret in README.md

[airadier]

What this PR does / why we need it:

Fix wrong name of global.sysdig.existingAccessKeySecret (should be accessKeySecret)

Checklist

[Place an '[x]' (no spaces) in all applicable fields. Please remove unrelated fields.]

• Title of the PR starts with type and scope, (e.g. feat(agent,node-analyzer,sysdig-deploy):)
• Chart Version bumped for the respective charts
• Variables are documented in the README.md (or README.tpl in some charts)
• Check GithubAction checks (like lint) to avoid merge-check stoppers
• All test files are added in the tests folder of their respective chart and have a "_test" suffix

Check Contribution guidelines in README.md for more insight.

[github-actions]

PR title does not comply with regex: ^(\w*)(?:\(([\w\$\.\,\-\*\s]*)\))?\:\s?(.*)$!
Check PR guidelines at https://github.com/sysdiglabs/charts/blob/master/README.md#pull-requests

All good!