chore(agent): update README.md

[yotakeu]

Change image.tag default value from 12.16.0 to 12.18.0.

What this PR does / why we need it:

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]

Hi @yotakeu. Thanks for your PR.

After inspecting your changes someone with write access to this repo needs
to approve and run the workflow with the following slash_command.

NOTE: the workflow triggered this way will only report the final status to this PR,
in order to check the logs please go to the Actions tab and look for approve-test-run-command.

• /approve-test-run sha=<short commit sha to test>: Run helm chart linting and testing workflow

[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

[mavimo]

@yotakeu thanks for your contribution!

/approve-test-run sha=72e5c502513f9e15b1492897f5648762abe9fa17

All good!