react-native-firebase

docs: use the example.com domain (#3979)

* use the example.com domain instead of gmail.

I don't know if sarah.lane wants here email on your docs, but we could change that to a more doc friendly user. if you go over to example.com they even add this on their website 


This domain is for use in illustrative examples in documents. You may use this domain in literature without prior coordination or asking for permission.

* fix typo

* update email to lower case

Co-authored-by: Elliot Hesp <elliot.hesp@gmail.com>