chore: set docs language to "en"

Author: lgray

Diff for: docs/source/conf.py

@@ -31,7 +31,7 @@
 release = coffea.__version__
 githash = subprocess.check_output(["git", "rev-parse", "HEAD"]).strip().decode("ascii")
 
-language = None
+language = "en"
 
 # -- General configuration ---------------------------------------------------