firebase-x, increment should have 2 parameters

[MauriceSchoenmakers]

I'm submitting a ... (check one with "x")
[x ] bug report
[ ] feature request
[ ] support request => Please do not submit support requests here, use one of these channels: https://forum.ionicframework.com/ or https://ionicworldwide.herokuapp.com/

Current behavior:
FirebaseX.incrementCounter has two parameters, now we get a type error when we pass 2 parameters

Expected behavior:
correct typescript type

Steps to reproduce:

Related code:

insert any relevant code here

Other information:

Ionic info: (run ionic info from a terminal/cmd prompt and paste output below):

insert the output from ionic info here

[danielsogl]

Feel free to adjust the interface and create a pull request. I will review your changes: https://github.com/danielsogl/awesome-cordova-plugins/blob/master/src/%40awesome-cordova-plugins/plugins/firebase-x/index.ts