Fix broken markup in CHANGELOG [ci skip]

Backport 378dd7a4.

https://github.com/rails/rails/blob/65bdd6ad085a02b976cd36f135c2a5ffb522e5a0/activesupport/CHANGELOG.md

activesupport/CHANGELOG.md

@@ -5,17 +5,17 @@
 
     Before:
 
-      'foobar'.truncate(5).frozen?
-      => true
-      'foobar'.truncate(6).frozen?
-      => false
+        'foobar'.truncate(5).frozen?
+        # => true
+        'foobar'.truncate(6).frozen?
+        # => false
 
     After:
 
-      'foobar'.truncate(5).frozen?
-      => false
-      'foobar'.truncate(6).frozen?
-      => false
+        'foobar'.truncate(5).frozen?
+        # => false
+        'foobar'.truncate(6).frozen?
+        # => false
 
     *Jordan Thomas*