Update New-AzureRmRoleDefinition.md

the command $role.AssignableScopes.Remove("/") | Out-Null does not work. 
changed it to $role.AssignableScopes.Clear()
as this clears the list of assigned scopes.

Author: HopGithub

src/ResourceManager/Resources/Commands.Resources/help/New-AzureRmRoleDefinition.md

@@ -98,7 +98,7 @@ PS C:\> $role = Get-AzureRmRoleDefinition -Name "Virtual Machine Contributor"
           PS C:\> $role.Actions.Add("Microsoft.Resources/subscriptions/resourceGroups/resources/read")
           PS C:\> $role.Actions.Add("Microsoft.Insights/alertRules/*")
           PS C:\> $role.Actions.Add("Microsoft.Support/*")
-          PS C:\> $role.AssignableScopes.Remove("/") | Out-Null
+          PS C:\> $role.AssignableScopes.Clear()
           PS C:\> $role.AssignableScopes.Add("/subscriptions/c276fc76-9cd4-44c9-99a7-4fd71546436e")
 
           PS C:\> New-AzureRmRoleDefinition -Role $role