Add another 404 check to SAML SSO doc

What does this MR do?

Adds a note to the 404 statement in https://docs.gitlab.com/ee/user/group/saml_sso/#users-receive-a-404 to outline another common reason for the 404 error: an incorrect SHA-1 fingerprint.

Related issues

Author's checklist

• Consider taking the GitLab Technical Writing Fundamentals course
• Follow the:
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  • Style Guide.
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• Request a review based on:
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If you are only adding documentation, do not add any of the following labels:

• ~"feature"
• ~"frontend"
• ~"backend"
• ~"bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

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