Update Azure DNS CNAME documentation to reflect reality

[ahoka]

From the code:

		"records": &schema.Schema{
			Type:     schema.TypeString,
			Optional: true,
			Elem:     &schema.Schema{Type: schema.TypeString},
			Set:      schema.HashString,
			Removed:  "Use `record` instead. This attribute will be removed in a future version",
		},

		"record": &schema.Schema{
			Type:     schema.TypeString,
			Required: true,

[tombuildsstuff]

Hi @ahoka!

Thanks for this PR - LGTM :)

[ghost]

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