Fix database event notation in fulltext-search example
Fix database event notation in fulltext-search example

When using $ notation, instead of {}, the whole database is returned
within the event instead of the data at the path of the event.

See [this Google
Group](https://groups.google.com/forum/#!topic/firebase-functions-truste
d-testers/PpJ4zdB6mAQ) for more information

firebase/functions-samples,firebase/functions-samples,Kevded/functions-samples,Kevded/functions-samples,firebase/functions-samples,Kevded/functions-samples,Kevded/functions-samples,firebase/functions-samples