[FIX] doc: fix link to our website

Remove wrong slash trailing

Now, in V11, matching of page are done on the exact name.

Previously, the error was transparent because we was using a
dedicated controller that allow trailing slash.
https://github.com/odoo/odoo/blob/11.0/odoo/http.py#L936

https://webmasters.googleblog.com/2010/04/to-slash-or-not-to-slash.html

    http://example.com/foo/ (with trailing slash, conventionally a directory)
    http://example.com/foo (without trailing slash, conventionally a file)

    Google treats each URL above separately     (and equally) regardless of
    whether it’s a file or a directory, or it contains a trailing slash or
    it doesn’t contain a trailing slash.

    Different content on / and no-/ URLs okay for Google, often less ideal for users

   Special case for homepage
       Rest assured that for your root URL specifically,
       http://example.com is equivalent to
       http://example.com/ and can’t be redirected even if you’re Chuck Norris.

doc/_extensions/odoo_ext/layout.html

@@ -102,8 +102,8 @@
                     <div>It's all about efficiency</div>
                   </div>
                   <ul>
-                    <li><a href="https://www.odoo.com/page/accounting/">Accounting</a></li>
-                    <li><a href="https://www.odoo.com/page/project-management/">Project</a></li>
+                    <li><a href="https://www.odoo.com/page/accounting">Accounting</a></li>
+                    <li><a href="https://www.odoo.com/page/project-management">Project</a></li>
                     <li class="dropdown">
                       <a href="#0" class="dropdown-toggle">Human Resources</a>
                       <ul>