Update location of RuboCop line length check

The new location was introduced in [RuboCop 0.78.0][1], via [this PR][2]

[1]: https://github.com/rubocop-hq/rubocop/blob/master/CHANGELOG.md#0780-2019-12-18
[2]: rubocop/rubocop#7542

.rubocop.yml

@@ -1,5 +1,5 @@
 ---
-Metrics/LineLength:
+Layout/LineLength:
   Max: 100
 Style/FrozenStringLiteralComment:
   Enabled: false