[gradle] Fixed a dependancy name

[daladim]

This fixes an "Error:Could not find com.afollestad.material-dialogs:core:0.8.1.0." when building from Android Studio.

By the way, it looks like this dependancy is now at version 2.0.0-rc5 (see https://github.com/afollestad/material-dialogs). Just for fun, I tried to set this version in build.gradle. Since it gave me the following error, I am not sure this would even improve MyOwnNotes, and I am quite new to Android apps after all, I gave up. Maybe this would be interesting? I'll let you choose.

Could not find androidx.appcompat:appcompat:1.0.2.
Required by:
    MyOwnNotes:app:unspecified > com.github.afollestad.material-dialogs:core:2.0.0-rc5

Thanks for MyOwnNotes anyway