Не вижу ошибки 
package com.javarush.task.task18.task1817;

/*
Пробелы
*/

import java.io.FileInputStream;
import java.io.IOException;

public class Solution {
    public static void main(String[] args) {
        double spacesCount, count, diff;
        spacesCount = count = 0.0;
        try (FileInputStream fis = new FileInputStream(args[0])) {
            int readed;
            do {
                readed = fis.read();
                if (readed == ' ') spacesCount++;
                     count++;
            } while (readed != -1);
        } catch (IOException e) {
            e.printStackTrace();
            }
        diff =  spacesCount * 100 / count;
        System.out.printf("%.2f", diff);
    }
}