Update 'backlog.md'

backlog.md

@@ -4,6 +4,8 @@
 - Stone Duality
 - Kan Extensions
 - Enriched Categories
+- Polynomial Functor
+- 2-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)*