Datamodel: Typo in string handling

[drsteve]

Code execution can fail due to a typo in the astype method, as seen at
https://github.com/spacepy/spacepy/blame/7b63f500b1ea55e503cac177f34788a1832e6577/spacepy/datamodel.py#L1589

This showed up when I was resurrecting an old code that used to work... however, I had updated SpacePy in the intervening ~3 years. It would take a little work to put a MWE together for the issue, but it is an obvious typo and fixing it allows my code to run/succeed. I believe this is encountered when trying to write an object to an HDF5 file.