Avoid shadowing builtin in `record_iternext` example

[davisagli]

The docstring for IStorageCurrentRecordIteration.record_iternext gives this example of usage:

        >>> next = None
        >>> while 1:
        ...     oid, tid, data, next = storage.record_iternext(next)
        ...     # do things with oid, tid, and data
        ...     if next is None:
        ...         break

This should probably use a different variable name since next is a builtin.