PEP 545: Update chapter name to builtins/functions (#5129)

Author: m-aciek

peps/pep-0545.rst

@@ -548,7 +548,7 @@ As soon as the translation hits:
 - 100% of bugs.html with proper links to the language repository
   issue tracker.
 - 100% of tutorial.
-- 100% of library/functions (builtins).
+- 100% of builtins/functions.
 
 the translation can be added to the language switcher.