docs: adjust types in autocomplete-display-example (angular#9240)

Author: rafaelss95

src/material-examples/autocomplete-display/autocomplete-display-example.ts

@@ -31,8 +31,8 @@ export class AutocompleteDisplayExample {
   ngOnInit() {
     this.filteredOptions = this.myControl.valueChanges
       .pipe(
-        startWith({} as User),
-        map(user => user && typeof user === 'object' ? user.name : user),
+        startWith<string | User>(''),
+        map(value => typeof value === 'string' ? value : value.name),
         map(name => name ? this.filter(name) : this.options.slice())
       );
   }
@@ -42,8 +42,8 @@ export class AutocompleteDisplayExample {
       option.name.toLowerCase().indexOf(name.toLowerCase()) === 0);
   }
 
-  displayFn(user: User): string {
-    return user ? user.name : user;
+  displayFn(user?: User): string | undefined {
+    return user ? user.name : undefined;
   }
 
 }