Mention Concatenate in ParamSpec < 3.10 infobox

Co-authored-by: Sebastian Rittau <[email protected]>

Author: smheidrich

docs/reference/generics.rst

@@ -665,7 +665,7 @@ for a more faithful type annotation:
 .. note::
 
    To use this feature on Python versions earlier than 3.10, you will need to
-   import ``ParamSpec`` from ``typing_extensions``.
+   import ``ParamSpec`` and ``Concatenate`` from ``typing_extensions``.
 
 Parameter specifications also allow you to describe decorators that
 alter the signature of the input function: