Fix up <Listing> usage in Ch. 18

Thanks to @LifeAdventurer (<[email protected]>) for flagging
this up on the tracking issue!

src/ch18-01-what-is-oo.md

@@ -44,7 +44,7 @@ on demand whenever anyone needs it. In other words, `AveragedCollection` will
 cache the calculated average for us. Listing 18-1 has the definition of the
 `AveragedCollection` struct:
 
-<Listing number="18-1" file-name="src/lib.rs" caption="Listing 18-1: An `AveragedCollection` struct that maintains a list of integers and the average of the items in the collection">
+<Listing number="18-1" file-name="src/lib.rs" caption="An `AveragedCollection` struct that maintains a list of integers and the average of the items in the collection">
 
 ```rust,noplayground
 {{#rustdoc_include ../listings/ch18-oop/listing-18-01/src/lib.rs}}
@@ -58,9 +58,7 @@ ensure that whenever a value is added or removed from the list, the average is
 also updated. We do this by implementing `add`, `remove`, and `average` methods
 on the struct, as shown in Listing 18-2:
 
-<Listing number="18-2" file-name="src/lib.rs" caption="Listing 18-2: Implementations of the public methods `add`, `remove`, and `average` on `AveragedCollection`">
-
-<span class="filename">Filename: src/lib.rs</span>
+<Listing number="18-2" file-name="src/lib.rs" caption="Implementations of the public methods `add`, `remove`, and `average` on `AveragedCollection`">
 
 ```rust,noplayground
 {{#rustdoc_include ../listings/ch18-oop/listing-18-02/src/lib.rs:here}}