Removing unused TimeGrain variable from example (Azure#15062)

You do not need the $TimeGrain entry in the Example 1 code block, as its not used anywhere.

Author: andyr8939

src/Monitor/Monitor/help/New-AzDiagnosticSetting.md

@@ -38,7 +38,6 @@ for
 
 ### Example 1
 ```powershell
-$TimeGrain=New-TimeSpan -Days 90
 $metric = New-AzDiagnosticDetailSetting -Metric -RetentionInDays 1 -RetentionEnabled -Category AllMetrics
 $log = New-AzDiagnosticDetailSetting -Log -RetentionInDays 1 -RetentionEnabled -Category Audit -Enabled
 $setting = New-AzDiagnosticSetting -TargetResourceId /subscriptions/XXXXXXXXXXXX/resourceGroups/XXXXXXXX/providers/Microsoft.Network/virtualNetworks/XXXXXXXX -Name diagnostic-test -WorkspaceId /subscriptions/XXXXXXXXXXXX/resourceGroups/XXXXXXXX/providers/Microsoft.OperationalInsights/workspaces/XXXXXXXXX -DedicatedLogAnalyticsDestinationType -Setting $log,$metric