Documentation: remaining PHP "var" declarations changed to "public"

Since PHP 4 isn't supported anymore, let's clean up these few
PHP "var" declarations which were remaining in the documentation.

According to my checks, there is no more PHP "var" left.
diff --git a/user_guide_src/source/general/models.rst b/user_guide_src/source/general/models.rst
index 87f63e4..2e1e025 100644
--- a/user_guide_src/source/general/models.rst
+++ b/user_guide_src/source/general/models.rst
@@ -18,9 +18,9 @@
 
 	class Blog_model extends CI_Model {
 
-	    var $title   = '';
-	    var $content = '';
-	    var $date    = '';
+	    public $title   = '';
+	    public $content = '';
+	    public $date    = '';
 
 	    function __construct()
 	    {