Update empty.rst

Correcting English.

doc/tests/empty.rst

@@ -5,7 +5,7 @@
 
 .. code-block:: jinja
 
-    {# evaluates to true if the foo variable is null, false, an empty array, or the empty string #}
+    {# evaluates to true if the foo variable is null, false, an empty array, or an empty string #}
     {% if foo is empty %}
         ...
     {% endif %}