Fix compile error.

Signed-off-by: Yufei Cai <yufei.cai@bosch.io>

thingsearch/service/src/main/java/org/eclipse/ditto/thingsearch/service/persistence/read/MongoThingsSearchPersistence.java

@@ -326,7 +326,7 @@ private static ThingId toThingId(final Document doc) {
 
     private static Optional<Instant> getModifiedTimestampOptional(final Document doc) {
         try {
-            final var path = List.of(PersistenceConstants.FIELD_THING, PersistenceConstants.FIELD_MODIFIED);
+            final var path = List.of(PersistenceConstants.FIELD_SORTING, PersistenceConstants.FIELD_MODIFIED);
             final var timestampString = doc.getEmbedded(path, String.class);
             if (timestampString != null) {
                 return Optional.of(Instant.parse(timestampString));