[rubygems/rubygems] Fix bad instance variable name

Recent changes made a warning while running specs show up for some
reason, and it revealed this error.

https://github.com/rubygems/rubygems/commit/bbf55de38e

spec/bundler/commands/doctor_spec.rb

@@ -79,7 +79,7 @@ RSpec.describe "bundle doctor" do
     it "exits with an error if home contains files that are not readable/writable" do
       expect { Bundler::CLI::Doctor.new({}).run }.not_to raise_error
       expect(@stdout.string).to include(
-        "Broken links exist in the Bundler home. Please report them to the offending gem's upstream repo. These files are:\n - #{@unwritable_file}"
+        "Broken links exist in the Bundler home. Please report them to the offending gem's upstream repo. These files are:\n - #{@broken_symlink}"
       )
       expect(@stdout.string).not_to include("No issues")
     end