Update generating-hash-for-a-token.md

[benhorton]

Fixed powershell block to base64 encode the output

Why:

Closes: #41519

What's being changed (if available, include any code snippets, screenshots, or gifs):

Change to 2 lines of code in the powershell code block
-$hashString = [System.BitConverter]::ToString($hash)
-$hashString.Replace('-', '')
+$hashString = [System.Convert]::ToBase64String($hash)
+$hashString

Check off the following:

• A subject matter expert (SME) has reviewed the technical accuracy of the content in this PR. In most cases, the author can be the SME. Open source contributions may require an SME review from GitHub staff.
• The changes in this PR meet the docs fundamentals that are required for all content.
• All CI checks are passing and the changes look good in the review environment.

[github-actions]

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Source	Review	Production	What Changed
admin/monitoring-activity-in-your-enterprise/reviewing-audit-logs-for-your-enterprise/identifying-audit-log-events-performed-by-an-access-token.md	ghec ghes@ 3.18 3.17 3.16 3.15 3.14	ghec ghes@ 3.18 3.17 3.16 3.15 3.14	from reusable
organizations/keeping-your-organization-secure/managing-security-settings-for-your-organization/identifying-audit-log-events-performed-by-an-access-token.md	ghec	ghec	from reusable

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