Hello: use const for magic strings

hello.go

@@ -2,11 +2,16 @@ package main
 
 import "fmt"
 
+const (
+	englishHelloPrefix      = "Hello "
+	englishDefaultHelloName = "world"
+)
+
 func Hello(name string) string {
 	if name == "" {
-		return "Hello world"
+		return englishHelloPrefix + englishDefaultHelloName
 	}
-	return fmt.Sprintf("Hello %s", name)
+	return fmt.Sprintf("%s%s", englishHelloPrefix, name)
 }
 
 func main() {