scrollspy: fix wrong activation of all nested links

Created by: mr-july

if the scrollspy applied to hierarchical navigation, then too many nav-links are activated. Example markup:

      <ul id="toc" class="nav nav-pills nav-stacked">
        <li>
          <a href="#v1" class="nav-link">Volume 1</a>
          <ul class="nav">
            <li>
              <a href="#ch-1-1" class="nav-link">Chapter 1.1</a>
            </li>
            <li>
              <a href="#ch-1-2" class="nav-link">Chapter 1.2</a>
            </li>
            <li>
              <a href="#ch-1-3" class="nav-link">Chapter 1.3</a>
            </li>
          </ul>
        </li>
        <li>
          <a href="#v2" class="nav-link">Volume 2</a>
          <ul class="nav">
            <li>
              <a href="#ch-2-1" class="nav-link">Chapter 2.1</a>
            </li>
            <li>
              <a href="#ch-2-2" class="nav-link">Chapter 2.2</a>
            </li>
          </ul>
        </li>
      </ul>

on activation of "Chapter 1.2" all siblings ("Chapter 1.1" and "Chapter 1.3") are also activated, because the selector

$('.nav-link[href="#ch-1-2"]').parents('li').find('.nav-links')

matches all ".nav-link"s under "#v1", so the correct selector should be

$('.nav-link[href="#ch-1-2"]').parents('li').find('> .nav-links')
                                                   ^
                                      only direct children of li

The illustration of this problem can be found on JSFiddle, and the solution (just patched version of bootstrap.min.js) is here.