docs(nvim): update configuration example

doc/nvim_context_vt.txt

@@ -66,7 +66,7 @@ To customize the behavior use the setup function:
           return nil
         end
         -- This is the standard text
-        return '--> ' .. utils.get_node_text(node)[1]
+        return opts.prefix .. ' ' .. utils.get_node_text(node)[1]
       end,
 
       -- Custom node validator callback