doc: missing (type := true) in reader monad example (#6196)

This PR adds missing `(types := true)` to `#reduce` example in [Readers
example](https://lean-lang.org/lean4/doc/monads/readers.lean.html).
Since [4.10](https://lean-lang.org/blog/2024-8-1-lean-4100/) the `(types
:= true)` is necessary for the `ReaderM Environment String` type to be
reduced into `Environment → String`.

doc/monads/readers.lean

@@ -139,7 +139,7 @@ You might be wondering, how does the context actually move through the `ReaderM`
 add an input argument to a function by modifying its return type?  There is a special command in
 Lean that will show you the reduced types:
 -/
-#reduce ReaderM Environment String   -- Environment → String
+#reduce (types := true) ReaderM Environment String   -- Environment → String
 /-!
 And you can see here that this type is actually a function!  It's a function that takes an
 `Environment` as input and returns a `String`.
@@ -196,4 +196,4 @@ entirely.
 
 Now it's time to move on to [StateM Monad](states.lean.md) which is like a `ReaderM` that is
 also updatable.
--/
+-/