Fix the URL for previewing Markdown changes.

site/dev/tools/markdown.md

 Markdown
 ========
 
 This site can handle normal MarkDown and many common extensions. To learn
 how the following is done please see the [source for this page](./markdown.md).
 Any file you put under `/site/` that has the extension `.md` will be processed
 as MarkDown. All other files will be served directly. For example, images
 can be added and they will be served correctly and referenced from within MarkDown files.
 
 When preparing for a code review of site docs you can get a preview of how the
 page will render by visiting the skia.org site and add a query parameter `cl`
 with the value of the Reitveld issue id:
 
-    https://skia.org?cl=REITVELD_ISSUE_NUMBER
+    https://skia.org/path/to/markdown-file?cl=REITVELD_ISSUE_NUMBER
 
 You can also run a local copy of the documentation server, which will allow
 you to preview changes much quicker. You must have [Go](https://golang.org)
 installed on your computer, which you will have if you are running on a Google
 corporate workstation. Run:
 
     go get skia.googlesource.com/buildbot.git/doc/go/docserver
 
 And then **from within** the directory of your local Git checkout of Skia run:
 
@@ skipping 367 matching lines @@
 
 Output:
 
     <p>If you want your page to validate under XHTML 1.0 Strict,
     you've got to put paragraph tags in your blockquotes:</p>
     
     <pre><code>&lt;blockquote&gt;
         &lt;p&gt;For example.&lt;/p&gt;
     &lt;/blockquote&gt;
     </code></pre>