Error message language: call them system specs (#2053)

Author: olleolleolle

lib/rspec/rails/example/system_example_group.rb

@@ -57,7 +57,7 @@ def app
             System test integration requires Rails >= 5.1 and has a hard
             dependency on a webserver and `capybara`, please add capybara to
             your Gemfile and configure a webserver (e.g. `Capybara.server =
-            :webrick`) before attempting to use system tests.
+            :webrick`) before attempting to use system specs.
           """.gsub(/\s+/, ' ').strip
         end