composer.json: require @alpha of fences 8.2

[JPustkuchen]

"require": {
"drupal/fences": "^8.2"
}

should be

"require": {
"drupal/fences": "^2.0@alpha"
}

(Minimum 2.x alpha version and don't use Drupal 8.x prefix, see https://www.drupal.org/project/fences)
There is no stable version existing yet and if you don't lower the general stability requirements composer will fail with:

Your requirements could not be resolved to an installable set of packages.

Problem 1
- Installation request for drupal/fences_block dev-master -> satisfiable by drupal/fences_block[dev-master].
- drupal/fences_block dev-master requires drupal/fences ^8.2 -> no matching package found.

It would be great if you could change this. Thank you!

[JPustkuchen]

I created a pull request: #2