Improve startup probe hint about progress deadline

[ReToCode]

Proposed Changes

• Improves the hint about progress deadline seconds in startup probe docs

[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

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[ReToCode]

/cherry-pick release-1.15

[knative-prow-robot]

@ReToCode: once the present PR merges, I will cherry-pick it on top of release-1.15 in a new PR and assign it to you.

In response to this:

/cherry-pick release-1.15

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-sigs/prow repository.

[ReToCode]

/assign @mgencur

[mgencur]

/lgtm

[knative-prow-robot]

@ReToCode: new pull request created: #6092

In response to this:

/cherry-pick release-1.15

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-sigs/prow repository.