check_certificate_expiration_command.ex: squash a compiler warning

(cherry picked from commit d0b66b8)

Author: michaelklishin

deps/rabbitmq_cli/lib/rabbitmq/cli/diagnostics/commands/check_certificate_expiration_command.ex

@@ -77,7 +77,7 @@ defmodule RabbitMQ.CLI.Diagnostics.Commands.CheckCertificateExpirationCommand do
   end
 
   def unit_label(1, unit) do
-    unit |> String.slice(0..-2)
+    unit |> String.slice(0..-2//-1)
   end
 
   def unit_label(_within, unit) do