Fix pay-per-use description in homepage

[guzalv]

"meaning that companies only use what they pay for" -> "meaning that companies only pay for what they use".

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: guzalv / name: Guzman (ed66fd0)

[knative-prow]

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

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[dprotaso]

/assign @Leo6Leo @Cali0707 (eventing book store app)

[dprotaso]

/lgtm
/approve
/cherry-pick release-1.17

[knative-prow-robot]

@dprotaso: once the present PR merges, I will cherry-pick it on top of release-1.17 in a new PR and assign it to you.

In response to this:

/lgtm
/approve
/cherry-pick release-1.17

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[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

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

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[knative-prow-robot]

@dprotaso: new pull request created: #6221

In response to this:

/lgtm
/approve
/cherry-pick release-1.17

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.