iab-vpaid: Correct AdVideoMidPoint to AdVideoMidpoint (lowercase 'p'). (#42792)

types/iab-vpaid/iab-vpaid-tests.ts

@@ -6,5 +6,8 @@ if (window.getVPAIDAd) {
     ad.subscribe((url, id, playerHandles) => {
         /* Handle click */
     }, 'AdClickThru');
+    ad.subscribe(() => {
+        /* Handle video midpoint */
+    }, 'AdVideoMidpoint');
     ad.startAd();
 }

types/iab-vpaid/index.d.ts

@@ -39,7 +39,7 @@ declare namespace iab.vpaid {
         | 'AdVolumeChange'
         | 'AdVideoStart'
         | 'AdVideoFirstQuartile'
-        | 'AdVideoMidPoint'
+        | 'AdVideoMidpoint'
         | 'AdVideoThirdQuartile'
         | 'AdVideoComplete'
         | 'AdUserAcceptInvitation'