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

## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have helpful link attached, see `contributing.md`

## Documentation / Examples

- [ ] Make sure the linting passes by running `pnpm lint`
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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')