Change generated code comment to match common Go

According to https://golang.org/s/generatedcode the generated
Go code should have a comment matches the following regexp:
```
^// Code generated .* DO NOT EDIT\.$
```

Author: alexandear

bool.go

@@ -103,7 +103,7 @@ func run(args []string) error {
 		return fmt.Errorf("reformat source: %v", err)
 	}
 
-	io.WriteString(w, "// @generated Code generated by gen-atomicint.\n\n")
+	io.WriteString(w, "// Code generated by gen-atomicint. DO NOT EDIT.\n\n")
 	_, err = w.Write(bs)
 	return err
 }

duration.go

@@ -190,7 +190,7 @@ func run(args []string) error {
 		return fmt.Errorf("reformat source: %v", err)
 	}
 
-	io.WriteString(w, "// @generated Code generated by gen-atomicwrapper.\n\n")
+	io.WriteString(w, "// Code generated by gen-atomicwrapper. DO NOT EDIT.\n\n")
 	_, err = w.Write(bs)
 	return err
 }