Update key-concepts.md

adding AT&T Detroit location

Author: reemas-new

articles/public-multi-access-edge-compute-mec/key-concepts.md

@@ -53,6 +53,7 @@ Every Azure public MEC site is associated with a parent Azure region. This regio
 | -------------- | --------------------- | ------------ | ------------- |
 | AT&T | ATT Atlanta A | attatlanta1 | East US 2 |
 | AT&T | ATT Dallas A | attdallas1 | South Central US |
+| AT&T | ATT Detroit A | attdetroit1 | Central US |
 
 ## Azure services