fix(ChatContent): fixed text color error when the role is system (#4112)

Author: anlyyao

packages/pro-components/chat/chat-content/chat-content.less

@@ -14,7 +14,8 @@
   box-sizing: border-box;
   width: fit-content;
 
-  &__user {
+  &__user,
+  &__system {
     color: @chat-content-user-text-color;
 
     ._pre {