Use h2 headings for Module::Concerning [ci-skip]

For SEO purposes, there should only be one `<h1>` per page, and the edge
version of SDoc will provide one using the module name if none are
present in documentation.

activesupport/lib/active_support/core_ext/module/concerning.rb

@@ -3,7 +3,7 @@
 require "active_support/concern"
 
 class Module
-  # = Bite-sized separation of concerns
+  # == Bite-sized separation of concerns
   #
   # We often find ourselves with a medium-sized chunk of behavior that we'd
   # like to extract, but only mix in to a single class.
@@ -18,9 +18,9 @@ class Module
   # with a comment, as a least-bad alternative. Using modules in separate files
   # means tedious sifting to get a big-picture view.
   #
-  # = Dissatisfying ways to separate small concerns
+  # == Dissatisfying ways to separate small concerns
   #
-  # == Using comments:
+  # === Using comments:
   #
   #   class Todo < ApplicationRecord
   #     # Other todo implementation
@@ -37,7 +37,7 @@ class Module
   #       end
   #   end
   #
-  # == With an inline module:
+  # === With an inline module:
   #
   # Noisy syntax.
   #
@@ -61,7 +61,7 @@ class Module
   #     include EventTracking
   #   end
   #
-  # == Mix-in noise exiled to its own file:
+  # === Mix-in noise exiled to its own file:
   #
   # Once our chunk of behavior starts pushing the scroll-to-understand-it
   # boundary, we give in and move it to a separate file. At this size, the
@@ -75,7 +75,7 @@ class Module
   #     include TodoEventTracking
   #   end
   #
-  # = Introducing Module#concerning
+  # == Introducing Module#concerning
   #
   # By quieting the mix-in noise, we arrive at a natural, low-ceremony way to
   # separate bite-sized concerns.