Change from PoP to pop.

_config.yml

@@ -10,7 +10,7 @@ description: Principles of Programming Group at Carnegie Mellon Univeristy
 footer_text:
 
 url: https://probsys.github.io/  # the base hostname & protocol for your site
-baseurl: /PoP  # the subpath of your site, e.g. /blog/
+baseurl: /pop  # the subpath of your site, e.g. /blog/
 
 # -----------------------------------------------------------------------------
 # Social integration