Update doc/advanced-topics.md

Co-Authored-By: Fermín Galán Márquez <fgalan@users.noreply.github.com>

Author: jason-fox

doc/advanced-topics.md

@@ -212,7 +212,7 @@ events in the IoT Agent with the configured type name will be marked as events.
 
 This plugin processes the entity attributes looking for a `TimeInstant` attribute. If one is found, for NGSI-v1/NGSIv2,
 the plugin adds a `TimeInstant` attribute as metadata for every other attribute in the same request. With NGSI-LD, the
-Standard `observedAt` metadata attribute is used instead.
+Standard `observedAt` property-of-a-property is used instead.
 
 ##### Expression Translation plugin (expressionTransformation)