Fixed typo in chrom spec

Sorry- rushed the last commit/fix

chrom should not be in inverted commas...
I don't mind which way you do this, but if you make this change you should also change the docstring for the 'identifier' argument, currently it says:

"An array of SNP identifiers. If not provided, identifiers will be created based on the variant index, e.g., ‘snp1’, ‘snp2’, etc."