[📚] Old Doc doesn't have sum() and average() example for flutter.

[TheNoumanDev]

Documentation of aggregate functions is old, PR for sum() and average() function is already merged in January and i thought it wasn't available but had to see code.

Link to doucmentation: https://firebase.google.com/docs/firestore/query-data/aggregation-queries#dart
Link to PR: #11757

[TarekkMA]

@TheNoumanDev Thank you for pointing this out. We will work on sorting this out.