fix display command parser

lib/ruby_algebra/command.rb

@@ -17,7 +17,8 @@ module RubyAlgebra
     end
 
     def to_s
-      result = @lhs.is_a?(Array) ? @lhs.join(', ') : @lhs.to_s
+      result = "Assignment command: "
+      result += @lhs.is_a?(Array) ? @lhs.join(', ') : @lhs.to_s
       result += ' := '
       result += @operand1.to_s
       case @operation
@@ -42,5 +43,11 @@ module RubyAlgebra
     def initialize(polynomial)
       @polynomial = polynomial
     end
+
+    def to_s
+      result = "Display command: "
+      result += @polynomial.to_s
+      result
+    end
   end
 end