fix web pubsub express d.ts file name (Azure#15008)

Author: bterlson

sdk/web-pubsub/web-pubsub-express/api-extractor.json

@@ -11,7 +11,7 @@
   "dtsRollup": {
     "enabled": true,
     "untrimmedFilePath": "",
-    "publicTrimmedFilePath": "./types/index.d.ts"
+    "publicTrimmedFilePath": "./types/web-pubsub-express.d.ts"
   },
   "messages": {
     "tsdocMessageReporting": {