Fix mistake in docs for keys filter

doc/filters/keys.rst

@@ -6,10 +6,10 @@ when you want to iterate over the keys of a sequence or a mapping:
 
 .. code-block:: twig
 
-    {% for key in [1, 2, 3, 4]|keys %}
+    {% for key in ['a', 'b', 'c', 'd']|keys %}
         {{ key }}
     {% endfor %}
-    {# outputs: 1 2 3 4 #}
+    {# outputs: 0 1 2 3 #}
 
     {% for key in {a: 'a_value', b: 'b_value'}|keys %}
         {{ key }}