docs(cdk/overlay): rename template variable from trigger to origin in example (#32821)

The #trigger template reference variable in the overlay basic example
is misleading because it suggests the element triggers the overlay
open/close behavior. In fact, it identifies the attachment point
(origin) for the connected overlay. Rename it to #origin for clarity.

Also fixes a typo in the comment ("it's" -> reworded).

Closes #31221

Author: yogeshwaran-c

src/components-examples/cdk/overlay/cdk-overlay-basic/cdk-overlay-basic-example.html

@@ -1,12 +1,12 @@
-<!-- This button triggers the overlay and is it's origin -->
-<button (click)="isOpen = !isOpen" type="button" cdkOverlayOrigin #trigger="cdkOverlayOrigin">
+<!-- This button is the origin to which the overlay is attached -->
+<button (click)="isOpen = !isOpen" type="button" cdkOverlayOrigin #origin="cdkOverlayOrigin">
   {{isOpen ? "Close" : "Open"}}
 </button>
 
 <!-- This template displays the overlay content and is connected to the button -->
 <ng-template
   cdkConnectedOverlay
-  [cdkConnectedOverlayOrigin]="trigger"
+  [cdkConnectedOverlayOrigin]="origin"
   [cdkConnectedOverlayOpen]="isOpen"
   (detach)="isOpen = false"
 >