doc: fix Interfaces example (#6499)

doc/docs.md

@@ -1440,8 +1440,12 @@ particularly useful for initializing a C library.
 ### Interfaces
 
 ```v
-struct Dog {}
-struct Cat {}
+struct Dog {
+    breed string
+}
+
+struct Cat {
+}
 
 fn (d Dog) speak() string {
     return 'woof'
@@ -1465,8 +1469,9 @@ fn perform(s Speaker) string {
     return s.speak()
 }
 
-dog := Dog{}
+dog := Dog{'Leonberger'}
 cat := Cat{}
+
 println(perform(dog)) // "woof"
 println(perform(cat)) // "meow"
 ```