Fix typo

Author: Derek Hower

lib/arch_obj_models/csr.rb

@@ -535,7 +535,7 @@ def wavedrom_desc(arch_def, effective_xlen, exclude_unimplemented: false, option
 
         desc["reg"] << { "bits" => n, type: 1 }
       end
-      if arch_def.partially_configured? && arch_def.optional_in_cfg?(arch_def)
+      if arch_def.partially_configured? && field.optional_in_cfg?(arch_def)
         desc["reg"] << { "bits" => field.location(arch_def, effective_xlen).size, "name" => field.name, type: optional_type }
       else
         desc["reg"] << { "bits" => field.location(arch_def, effective_xlen).size, "name" => field.name, type: 2 }