Update billing details for prescaling in Azure Firewall

[skmkzyk]

Clarified billing information regarding Capacity Unit Hour charges and default instance exclusions.

[prmerger-automator[bot]]

@skmkzyk : Thanks for your contribution! The author(s) and reviewer(s) have been notified to review your proposed change.

[learn-build-service-prod[bot]]

Learn Build status updates of commit fcf461d:

:white_check_mark: Validation status: passed

File	Status	Preview URL	Details
articles/firewall/prescaling.md	:white_check_mark:Succeeded

For more details, please refer to the build report.

[v-dirichards]

@sujamiya

Can you review the proposed changes?

Important: When the changes are ready for publication, adding a #sign-off comment is the best way to signal that the PR is ready for the review team to merge.

#label:"aq-pr-triaged" @MicrosoftDocs/public-repo-pr-review-team

[v-ccolin]

I sent an email to the content owner today.

@MicrosoftDocs/public-repo-pr-review-team

[v-dirichards]

I sent a Teams message to the content owner today.

@MicrosoftDocs/public-repo-pr-review-team

[sujamiya]

#sign-off

[learn-build-service-prod[bot]]

Learn Build status updates of commit 3a8a239:

:white_check_mark: Validation status: passed

File	Status	Preview URL	Details
articles/firewall/prescaling.md	:white_check_mark:Succeeded

For more details, please refer to the build report.

[ktoliver]

#sign-off

[prmerger-automator[bot]]

Invalid command: '#sign-off'. Only the assigned author of one or more file in this PR can sign off. @sujamiya