fix: type

Author: durran

src/change_stream.ts

@@ -189,7 +189,7 @@ export interface ChangeStreamDocumentCommon {
    * The format is "YYYY-MM-DD HH:MM.SS.millis".
    * @sinceServerVersion 6.0.0
    */
-  wallTime?: string;
+  wallTime?: Date;
 
   /**
    * The transaction number.

test/types/change_stream.test-d.ts

@@ -63,7 +63,7 @@ expectType<ResumeToken>(change._id);
 expectType<Timestamp | undefined>(change.clusterTime);
 expectType<number | undefined>(change.txnNumber); // Could be a Long if promoteLongs is off
 expectType<ServerSessionId | undefined>(change.lsid);
-expectType<string | undefined>(change.wallTime);
+expectType<Date | undefined>(change.wallTime);
 
 type CrudChangeDoc =
   | ChangeStreamInsertDocument<Schema> //  C