Fix Az.Accounts XML Comments

[ziyuezh576]

Description

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)
    • {Please put the link here}
  • the markdown help files have been regenerated using the commands listed here

[BethanyZhou]

Please see https://github.com/Azure/azure-powershell/pull/18663/files, Yuzhi modified it -> will throw a
why you roll back her change?

[ziyuezh576]

Because her change is wrong. What is the mean of "a token cache will throw a bool property"? Here should be an exception class of course, and the original exception class can be found with using a namespace at the top.

[BethanyZhou]

Make sense