Fix invalid array index generation in fml.coffee

Swapped the usage of `Math.round` with `Math.floor` to prevent the generation of an invalid array index and subsequent exception.


Example:

Assume Math.random() returns `.9` and the array length is 5.

`Math.round(.9 * 5)` yields 5, exceeding the bounds of the array, and triggers an exception.
`Math.floor(.9 * 5)` yields 4, a valid index and the script works as expected.