Change lint description for MergeableSelector

The grammar for this message was confusing and not helpful. Simplify it
to simply refer to the line of the rule the linter wants you to merge
the duplicate/nestable rule with.

Change-Id: I17578d72714af43d44831dac2b560181a467a79e
Reviewed-on: http://gerrit.causes.com/37606
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>