[index] Adjusting link to the PDF of this older version.

Author: Murilo Marinho

docs/source/index.rst

@@ -7,7 +7,7 @@
 
 .. hint::
 
-   You can download this tutorial as a `PDF <https://ros2-tutorial.readthedocs.io/_/downloads/en/latest/pdf/>`_ 📀.
+   You can download this tutorial as a `PDF <https://ros2-tutorial.readthedocs.io/_/downloads/en/humble/pdf/>`_ 📀.
 
 📖 **About this tutorial**