fix flare_dart path

[mdanics]

the previous path was relative and would result in an error if attempted to load the plugin via git

[luigi-rosso]

This is intentional, the master branch changes most often and we tend to reference it with a local clone instead of via GitHub pubspec reference. It's also the tree we branch from to do development work, which usually means we'll be working with both flare_dart and flare_flutter locally so having them on the filesystem makes it much easier.

But I do see how that's confusing for someone trying to use the latest bleeding edge. We have a note on the readme about it, but maybe we need to rename the branch/add a new one for this.

[mdanics]

Ah, missed that - my bad