chore: update description for coder_app url

This clarifies where users may be confused about how to set an external URL, as the current description only talks about internal URL usage. 

Closes #186

Author: matifali

provider/app.go

@@ -150,7 +150,7 @@ func appResource() *schema.Resource {
 			},
 			"url": {
 				Type: schema.TypeString,
-				Description: "A URL to be proxied to from inside the workspace. " +
+				Description: "An external url if \"external=true\" or a URL to be proxied to from inside the workspace. " +
 					"This should be of the form \"http://localhost:PORT[/SUBPATH]\". " +
 					"Either \"command\" or \"url\" may be specified, but not both.",
 				ForceNew:      true,