[css-color-6] Update slug and WPT for color-layers() (#10725)

Author: nt1m

css-color-6/Overview.bs

@@ -307,7 +307,7 @@ Contrasting Semi-transparent Colors</h4>
 █████ █▌  █▌   █▌   █████▌ █▌  █▌  ███▌ 
 -->
 
-Layering Multiple Colors: the ''color-layers()'' function {#colorlayers}
+Layering Multiple Colors: the ''color-layers()'' function {#color-layers}
 =========================================================================
 
 	The <dfn>color-layers()</dfn> [=functional notation=]
@@ -350,6 +350,12 @@ Layering Multiple Colors: the ''color-layers()'' function {#colorlayers}
 		and two colors, the first of which has non-unity alpha
 	</div>
 
+	<wpt>
+	  	color-layers-no-blend-mode.html
+		parsing/color-layers-invalid.html
+		parsing/color-layers-valid.html
+	</wpt>
+
 <!--
 ████████  ████████  ██████   ███████  ██       ██     ██ ████████
 ██     ██ ██       ██    ██ ██     ██ ██       ██     ██ ██
@@ -786,4 +792,4 @@ Changes {#changes}
 
 		</wpt>
 
-		-->
+		-->