The given version is correct: the comma is only required if the initializer is given. Your suggested version 

    typecode, [initializer]

implies that the comma is always required whether the initializer is given or not.

If we want to be absolutely pedantic, we could write:

    .. class:: array(typecode[, [initializer]])

as trailing commas are legal in function calls. But I don't think that makes for good documentation.