Fixed bug in XSD:boolean URI
Fixed bug in XSD:boolean URI

Boolean in dipper output currently look like `<http://www.ncbi.nlm.nih.gov/pubmed/11566854> :MONARCH_cliqueLeader "true"^^xsd: .` The python code was calling `XSD[bool]`, which works for some reason and just returns the namespace URI. Changed to `XSD['boolean']` so that the correct URI will be output as `xsd:boolean`.
monarch-initiative/dipper,TomConlin/dipper,monarch-initiative/dipper,TomConlin/dipper,monarch-initiative/dipper