Update 'IsAdmin' flag to use correct syntax 'Admin'

[lfoust]

The first example uses a flag -IsAdmin which does not exist. Updated to the correct syntax.

Description

The first example has a syntax error which uses the flag 'IsAdmin'. If a user copies and pastes this code they will end up with an error from powershell that the param 'IsAdmin' does not exist. Updated to the correct syntax which is just '-Admin'

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[azuresdkci]

Can one of the admins verify this patch?

[maddieclayton]

@azuresdkci Test this please

[maddieclayton]

@lfoust Thank you for your contribution!