Restore-AzSqlInstanceDatabase - fix "an" -> "a"

[Styxxy]

Description

Examples: fix "an deleted instance" to "a deleted instance"

I checked everything. I apart from the first to bullets, everything else seems not relevant for this PR as it is just some fixes in wording of help documents.
I hope it is ok like this.

Checklist

• I have read the Submitting Changes section of CONTRIBUTING.md
• The title of the PR is clear and informative
• The appropriate ChangeLog.md file(s) has been updated:
  • For any service, the ChangeLog.md file can be found at src/{{SERVICE}}/{{SERVICE}}/ChangeLog.md
  • A snippet outlining the change(s) made in the PR should be written under the ## Upcoming Release header -- no new version header should be added
• The PR does not introduce breaking changes
• If applicable, the changes made in the PR have proper test coverage
• For public API changes to cmdlets:
  • a cmdlet design review was approved for the changes in this repository (Microsoft internal only)
  • the markdown help files have been regenerated using the commands listed here

[adxsdkps]

Can one of the admins verify this patch?

[isra-fel]

It looks great. Thanks for your contribution 😀

[isra-fel]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 3 pipeline(s).

[isra-fel]

**Edit: ** it's OK now, right after I sent the comment 🤣

@dingmeng-xue the azure-powershell - security-tools job has already finished on ADO but the result was not sent to Github. I retried and it's the same.
Can you do a force merge? Thank you