fix(cdk/portal): write generic type for TemplateRef in the Template Portal code example (#23818)

Author: Sergey Vakhramov

src/cdk/portal/portal.md

@@ -70,7 +70,7 @@ Usage:
 ```
 
 ```ts
-@ViewChild('templatePortalContent') templatePortalContent: TemplateRef<unknow>;
+@ViewChild('templatePortalContent') templatePortalContent: TemplateRef<unknown>;
 
 ngAfterViewInit() {
   this.templatePortal = new TemplatePortal(