[css-backgrounds-4] Fixed id of Drop Shadows section w3c#8593

Author: SebastianZ

css-backgrounds-4/Overview.bs

@@ -739,7 +739,7 @@ The 'border-clip' properties</h3>
 		<p>The fragments are shown in red for illustrative purposes; they should be black in compliant UAs.
 	</div>
 
-<h2 id="misc">Drop Shadows</h2>
+<h2 id="drop-shadows">Drop Shadows</h2>
 
 <h3 id="box-shadow-color">Coloring shadows: the 'box-shadow-color' property</h3>