Correct comment in DBEngine Tests

The code in the DBEngine tests were updated but the comment explaining the test
did not get updated. This change corrects the comment to match what the test
is doing.

Change-Id: I56b290c375f15eb2fba70634d1c7d4ab841af1fe

test/offline/db_engine_unit.js

@@ -216,9 +216,9 @@ describe('DBEngine', /** @suppress {accessControls} */ function() {
     /** @type {number} */
     var id = db.reserveManifestId();
 
-    // Create a manifest with two streams. One video and one audio. When db
+    // Create a manifest with four streams. Two video and two audio. When db
     // engine recreates the variant ids, it should pair them together into
-    // a variant.
+    // four variants.
 
     /** @type {shakaExtern.ManifestDB} */
     var originalManifest = createManifest(id);