겨우 찾아서 고칠 수 있었다.
<%@ page language="java" contentType="text/html; charset=UTF-8"
pageEncoding="UTF-8"%>
<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width= device-width", initial-scale="1">
<link rel="stylesheet" href="css/bootstrap.css">
<title>JSP 게시판 웹 사이트</title>
</head>
<body>
<nav class="navbar navbar-default">
<div class ="navbar-header">
<!-- 햄버거 모양 -->
<button type="button" class="navbar-toggle collapsed" data-toggle="collapse"
data-target="#bs-example-navbar-collapse-1"aria-expanded="false">
<span class ="icon-bar"></span>
<span class ="icon-bar"></span>
<span class ="icon-bar"></span>
</button>
<a class="navbar-brand"href="main.jsp">JSP웹사이트 게시판</a>
</div>
<div class="collapse navbar-collapse" id="bs-example-navbar-collapse-1">
<ul class="nav navbar-nav">
<li><a href="main.jsp">메인</a></li>
<li><a href="bbs.jsp">게시판</a></li>
</ul>
<ul class="nav navbar-nav navbar-right">
<li class="dropdown">
<a href="#" class="dropdown-toggle" data-toggle="dropdown"
role="button" aria-haspopup="true" aria-expanded="false">
접속하기<span class="caret"></span></a>
<!-- 로그인과 회원가입 -->
<ul class="dropdown-menu">
<li class="active"><a href="login.jsp">로그인</a></li>
<li><a href="join.jsp">회원가입</a></li>
</ul>
</li>
</ul>
</div>
</nav>
<div class="container"> <!-- 감싸기 -->
<div class="col-lg-4"></div>
<div class="col-lg-4">
<div class="jumbotron" style="padding-top: 20px;">
<form method="post" action="LoginAction.jsp">
<!-- 안보이게 접속 -->
<h3 style="text-align; center;">로그인 화면</h3>
<div class="form-group">
<input type="text" class="form-control" placeholder="아이디" name="userID" maxlength="20">
</div>
<div class="form-group">
<input type="password" class="form-control" placeholder="비밀번호" name="userPassword" maxlength="20">
</div>
<input type="submit" class="btn btn-primary form-control" value="로그인">
</form>
</div>
</div>
<div class="col-lg-4"></div>
</div>
<script src="https://code.jquery.com/jquery-3.1.1.min.js"></script>
<script src="js/bootstrap.js"></script>
</body>
</html>