Clarify value type for SWC "Type" identifier

[eleftherioszisis]

The SWC field "Type", corresponding to column index 2, is defined as a positive integer in:

swc-specification/docs/source/swc.rst

Line 24 in 67a18f9

2 Type Type identifier. A positive integer.

And then below in the list of allowed types, there is also a 0 listed as a type for undefined structures:

swc-specification/docs/source/swc.rst

Line 37 in 67a18f9

undefined 0 Structure type unknown or unspecified

However, zero is not a positive integer, therefore there is a conflict between the definition of the field and its allowed values. I would assume that the field type is also allowed to be 0, but could you please verify it?