update prompt to fix field name includes dot(.)

Signed-off-by: Yulong Ruan <ruanyl@amazon.com>

src/plugins/vis_type_vega/public/text_to_vega.ts

@@ -19,6 +19,7 @@ ${ppl}
 
 The user's instruction is: ${input}
 
+when a field has a dot(.), you should escape the dot if the field is a single field. For example, if the field is "user.name", but the data is {"user.name": "John"}, the field should be escaped. But it should not be escaped if the data is {"user": {"name": "John"}}
 Just return the chart specification json based on Vega-Lite format.
 Just reply with the json based Vega-Lite object, do not include any other content in the reply.
 `;