[site24x7_schedule_maintenance] Use correct description / selection_type for argument monitor_groups

[dhoppe]

This argument needs to be specified if the selection_type is set to 1, not 3.
https://www.site24x7.com/help/api/#create-maintenance

The documentation mentions the argument maintenance_type, but it looks like the Terraform provider does not support it yet. Maybe you might want to mention that once ist the default and it would be really nice to be able to use this argument for real.

terraform-provider-site24x7/site24x7/common/schedule_maintenance.go

Lines 39 to 46 in 1d56f7a

	// As of now maintenance_type "3" is supported
	// "maintenance_type": {
	// Type: schema.TypeInt,
	// Optional: true,
	// Default: 3,
	// ValidateFunc: validation.IntInSlice([]int{3}),
	// Description: "Configuration for Once/Daily/Weekly/Monthly only maintenance. Default is 3 - Once. Refer https://www.site24x7.com/help/api/#schedule_maintenance_constants",
	// },