Change some 3.3 docs to be gender-neutral

Author: javiereguiluz

reference/configuration/security.rst

@@ -478,7 +478,7 @@ dn_string
 **type**: ``string`` **default**: ``{username}``
 
 This is the string which will be used as the bind DN. The ``{username}``
-placeholder will be replaced with the user-provided value (his login).
+placeholder will be replaced with the user-provided value (their login).
 Depending on your LDAP server's configuration, you may need to override
 this value.