docs: fix 404 link

Author: kenjis

user_guide_src/source/installation/upgrade_emails.rst

@@ -22,7 +22,7 @@ Upgrade Guide
 1. Within your class change the ``$this->load->library('email');`` to ``$email = service('email');``.
 2. From that on you have to replace every line starting with ``$this->email`` to ``$email``.
 3. The methods in the Email class are named slightly different. All methods, except for ``send()``, ``attach()``, ``printDebugger()`` and ``clear()`` have a ``set`` as prefix followed by the previous method name. ``bcc()`` is now ``setBcc()`` and so on.
-4. The config attributes in ``app/Config/Email.php`` have changed. You should have a look at the `Email Class Documentation </libraries/email.html#setting-email-preferences>`__ to have a list of the new attributes.
+4. The config attributes in **app/Config/Email.php** have changed. You should have a look at the :ref:`setting-email-preferences` to have a list of the new attributes.
 
 Code Example
 ============

user_guide_src/source/libraries/email.rst

@@ -44,6 +44,8 @@ Here is a basic example demonstrating how you might send email::
 
     $email->send();
 
+.. _setting-email-preferences:
+
 Setting Email Preferences
 =========================