✏ Fix broken link in alternatives.md (#5455)

Co-authored-by: Sebastián Ramírez <tiangolo@gmail.com>

docs/en/docs/alternatives.md

@@ -123,7 +123,7 @@ That's why when talking about version 2.0 it's common to say "Swagger", and for
 
 There are several Flask REST frameworks, but after investing the time and work into investigating them, I found that many are discontinued or abandoned, with several standing issues that made them unfit.
 
-### <a href="https://marshmallow.readthedocs.io/en/3.0/" class="external-link" target="_blank">Marshmallow</a>
+### <a href="https://marshmallow.readthedocs.io/en/stable/" class="external-link" target="_blank">Marshmallow</a>
 
 One of the main features needed by API systems is data "<abbr title="also called marshalling, conversion">serialization</abbr>" which is taking data from the code (Python) and converting it into something that can be sent through the network. For example, converting an object containing data from a database into a JSON object. Converting `datetime` objects into strings, etc.