Update ms-office-audit-source.md

[Hellfire4959]

During the on-boarding of the o365 sources, requesting global admin role account has raised security risk concerns. To avoid that it is recommended to use the global reader role

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[kimsauce]

thank you so much for the solid edit, @Hellfire4959 ! i made a few more changes and submitted it in a separate PR, and closing this one.