docs: fix minor error in the "Structural directives" guide (angular#37452)

The sample code used in this guide uses [class.od]="odd".
But, in another portion of the guide, [ngClass]="odd" is mentioned instead.

PR Close angular#37452

Author: ahasall

aio/content/guide/structural-directives.md

@@ -251,7 +251,7 @@ You enable these features in the string assigned to `ngFor`, which you write in
 
 Everything _outside_ the `ngFor` string stays with the host element
 (the `<div>`) as it moves inside the `<ng-template>`.
-In this example, the `[ngClass]="odd"` stays on the `<div>`.
+In this example, the `[class.odd]="odd"` stays on the `<div>`.
 
 
 </div>