docs(material/chips): fix image stretching in examples (#25116)

Fix image stretch issue raised here:
#19822

(cherry picked from commit 272a358)

src/components-examples/material/chips/chips-avatar/chips-avatar-example.html

@@ -1,20 +1,14 @@
 <mat-chip-list aria-label="Dog selection">
   <mat-chip>
-    <mat-chip-avatar>
-        <img src="https://material.angular.io/assets/img/examples/shiba1.jpg" alt="Photo of a Shiba Inu"/>
-    </mat-chip-avatar>
+    <img matChipAvatar src="https://material.angular.io/assets/img/examples/shiba1.jpg" alt="Photo of a Shiba Inu"/>
     Dog one
   </mat-chip>
   <mat-chip color="primary">
-    <mat-chip-avatar>
-        <img src="https://material.angular.io/assets/img/examples/shiba1.jpg" alt="Photo of a Shiba Inu"/>
-    </mat-chip-avatar>
+    <img matChipAvatar src="https://material.angular.io/assets/img/examples/shiba1.jpg" alt="Photo of a Shiba Inu"/>
     Dog two
   </mat-chip>
   <mat-chip color="accent">
-    <mat-chip-avatar>
-        <img src="https://material.angular.io/assets/img/examples/shiba1.jpg" alt="Photo of a Shiba Inu"/>
-    </mat-chip-avatar>
+    <img matChipAvatar src="https://material.angular.io/assets/img/examples/shiba1.jpg" alt="Photo of a Shiba Inu"/>
     Dog three
   </mat-chip>
 </mat-chip-list>