Documentation correction in xmlattribute.h.
Documentation correction in xmlattribute.h.

nickerso/libcellml,cellml/libcellml,MichaelClerx/libcellml,MichaelClerx/libcellml,hsorby/libcellml,cellml/libcellml,hsorby/libcellml,nickerso/libcellml,cellml/libcellml,cellml/libcellml,MichaelClerx/libcellml,nickerso/libcellml,hsorby/libcellml,nickerso/libcellml,hsorby/libcellml