Improve BacktraceCleaner code example [ci-skip]

With newer rubies, removing a prefix is much simpler.

activesupport/lib/active_support/backtrace_cleaner.rb

@@ -18,7 +18,7 @@ module ActiveSupport
   #
   #   bc = ActiveSupport::BacktraceCleaner.new
   #   root = "#{Rails.root}/"
-  #   bc.add_filter   { |line| line.start_with?(root) ? line.from(root.size) : line } # strip the Rails.root prefix
+  #   bc.add_filter   { |line| line.delete_prefix(root) } # strip the Rails.root prefix
   #   bc.add_silencer { |line| /puma|rubygems/.match?(line) } # skip any lines from puma or rubygems
   #   bc.clean(exception.backtrace) # perform the cleanup
   #