Correct example 17, auto->match-source. Fix w3c#6578

Author: svgeesus

css-masking-1/Overview.bs

@@ -1254,7 +1254,7 @@ The 'mask-type' property allows the author of the <a element>mask</a> element to
           &lt;/mask>
         &lt;/svg>
 
-        &lt;p style="mask-image: url(#mask); mask-mode: auto;">
+        &lt;p style="mask-image: url(#mask); mask-mode: match-source;">
           This is the masked content.
         &lt;/p>
     </code></pre>