Fixed serRoot action when sentence segmentation is gold

reading_machine/src/Action.cpp

@@ -689,7 +689,7 @@ Action Action::setRootUpdateIdsEmptyStackIfSentChanged()
   auto apply = [](Config & config, Action & a)
   {
     int lineIndex = config.getWordIndex();
-    int rootIndex = -1;
+    int rootIndex = lineIndex;
     int lastSentId = -1;
     int firstIndexOfSentence = lineIndex;