Modify the description about Max revision timeout seconds

[KAZYPinkSaurus]

This PR is related to knative/serving#14969 .

Because TerminationGracePeriodSeconds is more appropriate than terminationGraceTimeSeconds.
The document probably explain about fallowing value.
https://github.com/knative/serving/blob/6dc7097b740f6b0e6ae744abad5e84338a1b8066/config/core/deployments/activator.yaml#L124

I believe this PR makes this document more understandable.

Thank you.

Proposed Changes

• Modified terminationGraceTimeSeconds in the document of Max revision timeout seconds to terminationGracePeriodSeconds.

[linux-foundation-easycla]

• ✅login: KAZYPinkSaurus / (cdc3ed9)

The committers listed above are authorized under a signed CLA.

[knative-prow]

Welcome @KAZYPinkSaurus! It looks like this is your first PR to knative/docs 🎉

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[dprotaso]

/lgtm
/approve
/cherry-pick release-1.13

[knative-prow-robot]

@dprotaso: once the present PR merges, I will cherry-pick it on top of release-1.13 in a new PR and assign it to you.

In response to this:

/lgtm
/approve
/cherry-pick release-1.13

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[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dprotaso, KAZYPinkSaurus

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[knative-prow-robot]

@dprotaso: new pull request created: #5885

In response to this:

/lgtm
/approve
/cherry-pick release-1.13

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.