Update markdown docs to specify --local for the local devserver

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/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 -u skia.googlesource.com/buildbot.git/doc/go/docserver
 
 And then **from within** the directory of your local Git checkout of Skia run:
 
-    docserver --preview
+    docserver --preview --local
 
 Then visit http://localhost:8000 to preview your changes. There is no need to
 restart the server for file changes, but you will need to restart it if there
 are changes to the navigation menu, i.e. you add or remove a file and want it
 to appear in the navigation on the right hand side of the page.
 
 If port 8000 is unavailable on your machine you can set the port to use via
 the --port flag:
 
-    docserver --preview --port=:8002
+    docserver --preview --local --port=:8002
 
 METADATA
 --------
 
 By default all files and directories that appear in the same level are sorted
 alphabetically by file name in the navigation menu, with files appearing
 before directories. You can override this default behavior by adding a
 METADATA file to a directory. A METADATA file is a JSON file of the following
 format:
 
@@ skipping 366 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>