Fix dead anchor links

Author: l-vo

security/password_migration.rst

@@ -127,16 +127,18 @@ Upgrade the Password
 Upon successful login, the Security system checks whether a better algorithm
 is available to hash the user's password. If it is, it'll hash the correct
 password using the new hash. If you use a Guard authenticator, you first need to
-`provide the original password to the Security system <Provide the Password when using Guard>`_.
+:ref:`provide the original password to the Security system <provide-the-password-when-using-guard>`.
 
 You can enable the upgrade behavior by implementing how this newly hashed
 password should be stored:
 
-* `When using Doctrine's entity user provider <Upgrade the Password when using Doctrine>`_
-* `When using a custom user provider <Upgrade the Password when using a custom User Provider>`_
+* :ref:`When using Doctrine's entity user provider <upgrade-the-password-when-using-doctrine>`
+* :ref:`When using a custom user provider <upgrade-the-password-when-using-a-custom-user-provider>`
 
 After this, you're done and passwords are always hashed as secure as possible!
 
+.. _provide-the-password-when-using-guard:
+
 Provide the Password when using Guard
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
@@ -161,6 +163,8 @@ for this login request. This password is used in the migration process::
         }
     }
 
+.. _upgrade-the-password-when-using-doctrine:
+
 Upgrade the Password when using Doctrine
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
@@ -190,6 +194,8 @@ storing the newly created password hash::
         }
     }
 
+.. _upgrade-the-password-when-using-a-custom-user-provider:
+
 Upgrade the Password when using a Custom User Provider
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~