Update understand-security-groups.md

[Air-Git]

The text of Account Operators says they cannot manage Server Operators. Later it says that Account Operators is considered a service administrator group because it can modify Server Operators. This appears to be contradictory.

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[ttorble]

#assign: @robinharwood, @Xelu86

@robinharwood @Xelu86

Can you review the proposed changes?

#label:"aq-pr-triaged"
@MicrosoftDocs/public-repo-pr-review-team