added type to clientPromise in with-mongodb/lib (vercel#40339)

## Bug

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Author: S0UPernova

examples/with-mongodb/lib/mongodb.ts

@@ -8,7 +8,7 @@ const uri = process.env.MONGODB_URI
 const options = {}
 
 let client
-let clientPromise
+let clientPromise: Promise<MongoClient>
 
 if (!process.env.MONGODB_URI) {
   throw new Error('Please add your Mongo URI to .env.local')