Tweak vega-lite prompt (#930)

lumen/ai/prompts/VegaLiteAgent/main.jinja2

@@ -5,6 +5,6 @@ Generate the plot the user requested as a vega-lite specification.
 
 - The data will be provided separately. Never provide a data field.
 - Ensure you use the column names verbatim.
-- To sort the x axis, use `sort: x` or `sort: -x` for descending.
-- Integer columns like year (e.g., 2010, 2011) should be treated as quantitative, not temporal, but '2010-01' or '2011-01-01' should be treated as temporal.
+- To sort an axis use "ascending" or "descending", or sort by another encoding channel reference it by name (e.g. `sort: y` or `sort: -y` for descending order).
+- Integer columns like year (e.g., 2010, 2011) should be treated as ordinal, not numerical or temporal, but '2010-01' or '2011-01-01' should be treated as temporal.
 {% endblock %}