[rubygems/rubygems] Fix advice in bundle install --system deprecation

https://github.com/rubygems/rubygems/commit/59a66e3560

lib/bundler/cli.rb

@@ -250,10 +250,12 @@ module Bundler
     def install
       SharedHelpers.major_deprecation(2, "The `--force` option has been renamed to `--redownload`") if ARGV.include?("--force")
 
-      %w[clean deployment frozen no-prune path shebang system without with].each do |option|
+      %w[clean deployment frozen no-prune path shebang without with].each do |option|
         remembered_flag_deprecation(option)
       end
 
+      print_remembered_flag_deprecation("--system", "path.system", "true") if ARGV.include?("--system")
+
       remembered_negative_flag_deprecation("no-deployment")
 
       require_relative "cli/install"

spec/bundler/other/major_deprecation_spec.rb

@@ -377,7 +377,7 @@ RSpec.describe "major deprecations" do
       "no-prune" => ["no_prune", "true"],
       "path" => ["path", "'vendor/bundle'"],
       "shebang" => ["shebang", "'ruby27'"],
-      "system" => ["system", "true"],
+      "system" => ["path.system", "true"],
       "without" => ["without", "'development'"],
       "with" => ["with", "'development'"],
     }.each do |name, expectations|