allow spaces in array descriptors

Changing "~" to "::=" resulted in `<array descriptor>` confused with `<character class>` for inputs like `[value]` so I did it [as you suggested](https://groups.google.com/d/msg/marpa-parser/NO34L_xe-QU/rw7uUKxOsNYJ).

I was tempted to allow whitespace only for both square brackets (thus invalidating `[ values]` and `[values ]`) but then thought we have to be liberal in what we accept, as Mark Pilgrim aptly said. Also, allowing `[ values]` and `[values ]` is closer to the definition of whitespace discarding.