[bundler/bundler] Correct a rubygems test filter

It seems to pass under rubygems 2.5.2.

https://github.com/bundler/bundler/commit/062cd2be7c

spec/bundler/commands/exec_spec.rb

@@ -851,7 +851,7 @@ __FILE__: #{path.to_s.inspect}
         bundle :install, :system_bundler => true, :path => "vendor/bundler"
       end
 
-      it "overrides disable_shared_gems so bundler can be found", :ruby_repo, :rubygems => ">= 2.6.2" do
+      it "overrides disable_shared_gems so bundler can be found", :ruby_repo, :rubygems => ">= 2.5.2" do
         system_gems :bundler
         file = bundled_app("file_that_bundle_execs.rb")
         create_file(file, <<-RB)