docs: Fix minor grammatical error in operator manual overview

This changes the words administrator and operator to plural versions.

Checklist:

[x] Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this does not need to be in the release notes.
[x] The title of the PR states what changed and the related issues number (used for the release note).
[x] The title of the PR conforms to the Toolchain Guide
[x] I've included "Closes [ISSUE #]" or "Fixes [ISSUE #]" in the description to automatically close the associated issue.
[x] I've updated both the CLI and UI to expose my feature, or I plan to submit a second PR with them.
[x] Does this PR require documentation updates?
[x] I've updated documentation as required by this PR.
[x] I have signed off all my commits as required by DCO
[x] I have written unit and/or e2e tests for my change. PRs without these are unlikely to be merged.
[x] My build is green (troubleshooting builds).
[x] My new feature complies with the feature status guidelines.
[x] I have added a brief description of why this PR is necessary and/or what this PR solves.
[x] Optional. My organization is added to USERS.md.
[x] Optional. For bug fixes, I've indicated what older releases this fix should be cherry-picked into (this may or may not happen depending on risk/complexity).

argoproj / argo-cd