DOC: Fixing 'chunksize' parameter name typo io.rst
Fixing an error in the 'chunksize' parameter name for the io doc.  With the typo (parameter called 'chunsize'), the sample code from the documentation fails silently / confusingly - returns an iterator over individual dataframe elements rather than dataframe chunks.

@nesanders good point (and not why it was in a code-block originally, don't remember). (that of course is not caught in the doc build!)
