Error says Zkn, but check is for Zk

[marcfedorow]

riscv-config/riscv_config/isa_validator.py

Line 93 in d918906

err_list.append( "Zkn is a superset of Zbkb, Zbkc, Zbkx, Zkne, Zknd, Zknh, Zkn, Zkt, Zkr. In presence of Zkn the subsets must be ignored in the ISA string.")

Similar to #108