Update 'backlog.md'

backlog.md

@@ -3,6 +3,7 @@
 - Ends and Coends
 - Stone Duality
 - Kan Extensions
+- Enriched Categories
 
 ## Reading Stack Backlog
 - [Marx, Engels and the Abolition of the Family](https://www.csustan.edu/sites/default/files/History/Faculty/Weikart/Marx-Engels-and-the-Abolition-of-the-Family.pdf) *(added 5/3/2023)*