Error 404 ao gerar pdf no Exercício 6. | Jasper Reports: Relatórios com Java

Ao baixar o projeto e tentar gerar o PDF (aos 5:20 da aula 6), aparece o errro abaixo. Poderia ajudar na solução?

HTTP ERROR 404

Problem accessing /movimentacoes-web/contas. Reason:

    Not Found

<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Insert title here</title>
</head>
<body>
<form action="contas" method="POST">

    <input type="submit" value="Gera PDF" />

</form>
</body>
</html>

package br.com.caelum.relatorio.servlet;

import java.io.IOException;
import java.sql.Connection;
import java.sql.SQLException;
import java.util.HashMap;
import java.util.Map;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import br.com.caelum.relatorio.ConnectionFactory;
import br.com.caelum.relatorio.gerador.GeradorRelatorio;

@WebServlet("/contas")
public class RelatorioServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        try {

            String nome = request.getServletContext().getRealPath("/WEB-INF/jasper/contas_por_mes.jasper");
            Map<String, Object> parametros = new HashMap<String, Object>();
            Connection connection = new ConnectionFactory().getConnection();
            GeradorRelatorio gerador = new GeradorRelatorio(nome, parametros, connection);
            gerador.geraPDFParaOutputStream(response.getOutputStream());

            connection.close();
        } catch (SQLException e) {
            throw new ServletException(e);
        } 
    }
}