Fix duplicate ID (and ensuing bikeshed warning).

Author: dbaron

css-masking-1/Overview.html

@@ -94,7 +94,7 @@ <h2 class="no-num no-toc no-ref" id="contents">Table of Contents</h2>
       <li><a href="#the-clip-rule"><span class="secno">6.2</span> <span class="content">Winding Rules: the <span class="property">clip-rule</span> property</span></a>
      </ol>
     <li>
-     <a href="#masking0"><span class="secno">7</span> <span class="content">Positioned Masks</span></a>
+     <a href="#positioned-masks"><span class="secno">7</span> <span class="content">Positioned Masks</span></a>
      <ol class="toc">
       <li><a href="#the-mask-image"><span class="secno">7.1</span> <span class="content">Mask Image Source: the <span class="property">mask-image</span> property</span></a>
       <li><a href="#the-mask-mode"><span class="secno">7.2</span> <span class="content">Mask Image Interpretation: the <span class="property">mask-mode</span> property</span></a>
@@ -463,7 +463,7 @@ <h3 class="heading settled" data-level="6.2" id="the-clip-rule"><span class="sec
   &lt;rect clip-path="url(#MyClip)" clip-rule="evenodd" ... />
 &lt;/g></code></pre>
    </div>
-   <h2 class="heading settled" data-level="7" id="masking0"><span class="secno">7. </span><span class="content">Positioned Masks</span><a class="self-link" href="#masking0"></a></h2>
+   <h2 class="heading settled" data-level="7" id="positioned-masks"><span class="secno">7. </span><span class="content">Positioned Masks</span><a class="self-link" href="#positioned-masks"></a></h2>
    <h3 class="heading settled" data-level="7.1" id="the-mask-image"><span class="secno">7.1. </span><span class="content">Mask Image Source: the <a class="property" data-link-type="propdesc" href="#propdef-mask-image">mask-image</a> property</span><a class="self-link" href="#the-mask-image"></a></h3>
    <table class="def propdef" data-link-for-hint="mask-image">
     <tbody>

css-masking-1/Overview.src.html

@@ -349,7 +349,7 @@ <h3 id="the-clip-rule">Winding Rules: the 'clip-rule' property</h3>
 &lt;/g&gt;</code></pre>
 </div>
 
-<h2 id="masking">Positioned Masks</h2>
+<h2 id="positioned-masks">Positioned Masks</h2>
 
 <h3 id="the-mask-image">Mask Image Source: the 'mask-image' property</h3>