rename risc_value to register_value

almost to register, but it still carries that value

lib/risc/instructions/operator_instruction.rb

@@ -17,8 +17,8 @@ module Risc
       raise "unsuported operator :#{operator}:" unless Risc.operators.include?(operator)
       @left = left
       @right = right
-      raise "Not register #{left}" unless RiscValue.look_like_reg(left)
-      raise "Not register #{right}" unless RiscValue.look_like_reg(right)
+      raise "Not register #{left}" unless RegisterValue.look_like_reg(left)
+      raise "Not register #{right}" unless RegisterValue.look_like_reg(right)
     end
     attr_reader :operator, :left , :right