[ci skip] Fix rdoc markup

activerecord/lib/active_record/attribute_methods/serialization.rb

@@ -11,7 +11,7 @@ module ClassMethods
         # serialized object must be of that class on assignment and retrieval.
         # Otherwise <tt>SerializationTypeMismatch</tt> will be raised.
         #
-        # Empty objects as +{}+, in the case of +Hash+, or +[]+, in the case of
+        # Empty objects as <tt>{}</tt>, in the case of +Hash+, or <tt>[]</tt>, in the case of
         # +Array+, will always be persisted as null.
         #
         # Keep in mind that database adapters handle certain serialization tasks

activesupport/lib/active_support/hash_with_indifferent_access.rb

@@ -92,7 +92,7 @@ def self.[](*args)
     #   hash = ActiveSupport::HashWithIndifferentAccess.new
     #   hash[:key] = 'value'
     #
-    # This value can be later fetched using either +:key+ or +'key'+.
+    # This value can be later fetched using either +:key+ or <tt>'key'</tt>.
     def []=(key, value)
       regular_writer(convert_key(key), convert_value(value, for: :assignment))
     end