Fix document in control-plane/architecture-overview: "api" > "server"

[ShotaKitazawa]

What this PR does / why we need it:

fix document:
In architecture overview of control-plane, written as cache used by api. I think cache use by server is correct after PipeCD v0.8.0.

If I am wrong, please close this PR.

Which issue(s) this PR fixes:

None

Does this PR introduce a user-facing change?:

NONE

[pipecd-bot]

Welcome @ShotaKitazawa!

Thanks for your contributing. It looks like this is your first PR to this repository 🎉. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA). Please read the CLA and leave a /cla sign comment on this pull request to sign the CLA.

[ShotaKitazawa]

/cla sign

[pipecd-bot]

Thank you for signing the CLA. Your signing has been successfully committed.

[nghialv]

@ShotaKitazawa Thank you for your pull request.
/approve

[pipecd-bot]

This pull request is APPROVED by nghialv.

Approvers can cancel the approval by writing /approve cancel in a comment. Any additional commits also will change this pull request to be not-approved.

[nghialv]

/trigger presubmits

[pipecd-bot]

@nghialv: Your requested presubmits has been scheduled in response to this comment.

[pipecd-bot]

Code coverage for golang is 33.96%. This pull request does not change code coverage.