Edit sample YAML config for easier importing to web docs

[feorlen]

Small edits to server-config.yaml to better match web docs style. This makes it easier to include a sample config in the KES docs.

Closes minio/docs#1123

[feorlen]

@djwfyi adding you as reviewer to confirm edits before additional approvals.

[djwfyi]

One singular/plural mashup I want to figure out how to rephrase, the rest LGTM.

[feorlen]

@aead When you have a moment, appreciate a review of the edited comments in the example server-config.yaml file. Nothing should have changed in the meaning, only a bit better match to the web docs style so it's easier to copy into docs. Thanks!