Change configuration example to use LineLength

Using the `RuboCop` linter as an example wasn't very useful, as that
linter typically shouldn't be customized (even though it can be).

Switch the example to use `LineLength`, as that's a better example.

Change-Id: I76996d723168925b182a64cf892c776244a82a84
Reviewed-on: http://gerrit.causes.com/40445
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>