Be clear that the methods-and-traits exercise does not require generics (#2568)

When teaching the course, I got a little tripped up thinking students
would need to make the `VerbosityFilter` generic over `Logger`. Let's be
clearer that this is not required, and will be described later.

This also updates the generic-types slide to repeat the exercise,
completing that thought.

src/methods-and-traits/exercise.md

@@ -1,5 +1,5 @@
 ---
-minutes: 20
+minutes: 15
 ---
 
 # Exercise: Logger Trait
@@ -14,13 +14,14 @@ verbosity. Your task is to write a `VerbosityFilter` type that will ignore
 messages above a maximum verbosity.
 
 This is a common pattern: a struct wrapping a trait implementation and
-implementing that same trait, adding behavior in the process. What other kinds
-of wrappers might be useful in a logging utility?
+implementing that same trait, adding behavior in the process. In the "Generics"
+segment this afternoon, we will see how to make the wrapper generic over the
+wrapped type.
 
 ```rust,compile_fail
 {{#include exercise.rs:setup}}
 
-// TODO: Define and implement `VerbosityFilter`.
+// TODO: Implement the `Logger` trait for `VerbosityFilter`.
 
 {{#include exercise.rs:main}}
 ```

src/methods-and-traits/exercise.rs

@@ -26,13 +26,13 @@ impl Logger for StderrLogger {
         eprintln!("verbosity={verbosity}: {message}");
     }
 }
-// ANCHOR_END: setup
 
 /// Only log messages up to the given verbosity level.
 struct VerbosityFilter {
     max_verbosity: u8,
     inner: StderrLogger,
 }
+// ANCHOR_END: setup
 
 impl Logger for VerbosityFilter {
     fn log(&self, verbosity: u8, message: &str) {