Update 'backlog.md'

backlog.md

@@ -6,6 +6,7 @@
 - Enriched Categories
 - Polynomial Functor
 - 2-Categories
+- Reflective subcategory
 
 ## 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)*