clarified concept instance vs class

Author: alex-quin-gabriel

docs/concepts/concept.md

@@ -1,6 +1,6 @@
 ## Types
 ### **concept** `$tType` { #concept data-toc-label='concept' }
-The class of a [phenomenon](phenomenon.md) in the agent's world model. 
+An instance of a class of [phenomenon](phenomenon.md) in the agent's world model. 
 
 A [concept](#concept) is a [property](property.md) of a [modelet](modelet.md).