Fix a non-compiling example in the type abstractions docs

This patch adds a missing Show constraint to a code example in the User Guide's type abstractions docs to fix issue #25422.

docs/users_guide/exts/type_abstractions.rst

@@ -54,7 +54,7 @@ part of the type argument using type constructors.
 
 For a somewhat-contrived example::
 
-    foo :: (Num a) => Maybe [a] -> String
+    foo :: (Show a, Num a) => Maybe [a] -> String
     foo (Nothing @[t]) = show (0 :: t)
     foo (Just @[t] xs) = show (sum xs :: t)