IndexForm roundtrip to json fails for i32/u32

[nsmith-]

Because of the typo here: https://github.com/scikit-hep/awkward-1.0/blob/6db9f5996d09e7ee04e7b94100d0cdccfe1c2a07/src/libawkward/Index.cpp#L23
making a ListOffsetArray32 form doesn't work:

>>> awkward1.forms.ListOffsetForm("i32", awkward1.forms.EmptyForm())
{
    "class": "UnrecognizedListOffsetArray",
    "offsets": "i8",
    "content": {
        "class": "EmptyArray"
    }
}

I can make the patch.