fix register use in putstring

was off by one, the syscall is write 
and the first arg is file_descriptor
ie 1 == stdout

lib/risc/interpreter.rb

@@ -275,7 +275,8 @@ module Risc
     end
 
     def handle_putstring
-      str = get_register( :r0 ) # should test length, ie r2
+      # should test length, syscall_3  (syscall_1 is file_descriptor, ie stdout)
+      str = get_register( std_reg(:syscall_2) )
       case str
       when Symbol
         @stdout << str.to_s

lib/risc/platform.rb

@@ -40,6 +40,8 @@ module Risc
         :r0
       when :syscall_2
         :r1
+      when :syscall_3
+        :r2
       when :saved_message
         :r14
       else