dashboard/app: fix comment reference to ExternalConfig struct

Commit b190f06 ("dashboard/app: fix testing for go1.11 runtime")
removed the unused ExternalConfig struct.

However, config.go still refers to ExternalConfig in a comment explaining
how it can be used to attach to external reporting systems. Change this
to say the user can implement their own type if they need.

Signed-off-by: Andrew Donnellan <ajd@linux.ibm.com>

Author: ajdlinux

dashboard/app/config.go

@@ -301,7 +301,7 @@ type Reporting struct {
 	Embargo time.Duration
 	// Type of reporting and its configuration.
 	// The app has one built-in type, EmailConfig, which reports bugs by email.
-	// And ExternalConfig which can be used to attach any external reporting system (e.g. Bugzilla).
+	// The user can implement other types to attach to external reporting systems (e.g. Bugzilla).
 	Config ReportingType
 	// List of labels to notify about (keys are strings of form "label:value").
 	// The value is the string that will be included in the notification message.