docs: add sub section title

kenjis · kenjis · commit 019ebeaed681 · 2023-06-12T08:47:53.000+09:00
diff --git a/user_guide_src/source/incoming/routing.rst b/user_guide_src/source/incoming/routing.rst
@@ -114,6 +114,9 @@ Or using ``use`` keyword:
 .. literalinclude:: routing/014.php
    :lines: 2-
 
+Array Callable Syntax and Placeholders
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
 If there are placeholders, it will automatically set the parameters in the specified order:
 
 .. literalinclude:: routing/015.php
