Update app.go

Co-authored-by: Dan Buch <dan@meatballhat.com>

app.go

@@ -306,7 +306,8 @@ func (a *App) RunContext(ctx context.Context, arguments []string) (err error) {
 }
 
 // This is a stub function to keep public API unchanged from old code
-// No one should really use this. Always use a.Run to execute app
+// 
+// Deprecated: use App.Run or App.RunContext
 func (a *App) RunAsSubcommand(ctx *Context) (err error) {
 	return a.RunContext(ctx.Context, ctx.Args().Slice())
 }