test: Fix flaky concurrent test in telemetry buffer

Author: sl0thentr0py

sentry-ruby/spec/support/shared_examples_for_telemetry_event_buffers.rb

@@ -67,7 +67,7 @@
 
       threads = 3.times.map do
         Thread.new do
-          (20..30).to_a.sample.times { subject.add_item(event) }
+          (21..30).to_a.sample.times { subject.add_item(event) }
         end
       end