docs: fix title underline

kenjis · kenjis · commit 9f2d3ced709e · 2022-04-06T20:23:33.000+09:00
factories.rst:13: WARNING: Title underline too short.
diff --git a/user_guide_src/source/concepts/factories.rst b/user_guide_src/source/concepts/factories.rst
@@ -10,7 +10,7 @@ Introduction
 ============
 
 What are Factories?
-------------------
+-------------------
 
 Like :doc:`./services`, **Factories** are an extension of autoloading that helps keep your code
 concise yet optimal, without having to pass around object instances between classes.
