fix(): return scanner.Err() instead of nil (#3)

The code description in the book does not match the example in github.

pkg/sum/sum.go

@@ -204,7 +204,7 @@ func Sum5(fileName string) (ret int64, err error) {
 
 		ret += num
 	}
-	return ret, nil
+	return ret, scanner.Err()
 }
 
 func Sum5_line(fileName string) (ret int64, err error) {