Change the value of "pre" button in Markdown toolbar from "~~~" to "```" (#22843).

Patch by Go MAEDA.


git-svn-id: http://svn.redmine.org/redmine/trunk@17295 e93f8b46-1217-0410-a6f0-8f06a7374b81

public/help/nl/wiki_syntax_detailed_markdown.html

@@ -286,9 +286,9 @@ It can be expanded by clicking a link.
     <p>You can highlight code at any place that supports wiki formatting using this syntax (note that the language name or alias is case-insensitive):</p>
 
 <pre>
-~~~ ruby
+``` ruby
   Place your code here.
-~~~
+```
 </pre>
 
     <p>Example:</p>