Fix typo in doc: build-in -> built-in
diff --git a/docs/reference.rst b/docs/reference.rst
index e41141b..3f74849 100644
--- a/docs/reference.rst
+++ b/docs/reference.rst
@@ -80,7 +80,7 @@
 
 .. doxygenfunction:: add_ostream_redirect
 
-Python build-in functions
+Python built-in functions
 =========================
 
 .. doxygengroup:: python_builtins