Add another example

Author: benjie

spec/Section 3 -- Type System.md

@@ -1781,6 +1781,7 @@ Following are examples of input coercion with various list types and values:
 | `[Int]`       | `null`           | `null`                      |
 | `[[Int]]`     | `[[1], [2, 3]]`  | `[[1], [2, 3]]`             |
 | `[[Int]]`     | `[1, 2, 3]`      | `[[1], [2], [3]]`           |
+| `[[Int]]`     | `[1, [2], 3]`    | `[[1], [2], [3]]`           |
 | `[[Int]]`     | `[1, null, 3]`   | `[[1], null, [3]]`          |
 | `[[Int]]`     | `[[1], ["b"]]`   | Error: Incorrect item value |
 | `[[Int]]`     | `1`              | `[[1]]`                     |