Fix typo of backtick instead of quote

https://bugzilla.redhat.com/show_bug.cgi?id=1789153

Followup to ManageIQ/manageiq#19701

Seen when run locally or in travis output:

```
472.50s$ bundle exec rake
** ManageIQ master, codename: Jansa
[Coveralls] Set up the SimpleCov formatter.
[Coveralls] Using SimpleCov's 'rails' settings.
** ManageIQ master, codename: Jansa
/home/travis/build/ManageIQ/manageiq-automation_engine/vendor/bundle/gems/rspec-core-3.9.1/exe/rspec: No such file or directory - does
/home/travis/build/ManageIQ/manageiq-automation_engine/vendor/bundle/gems/rspec-core-3.9.1/exe/rspec: No such file or directory - does
Randomized with seed 8807
....
```

spec/engine/miq_ae_method_service/miq_ae_service_model_base_spec.rb

@@ -122,11 +122,11 @@
     let(:service_model) { MiqAeMethodService::MiqAeServiceManageIQ_Providers_InfraManager_Vm }
 
     describe '.ar_subclass_associations' do
-      it `does not return the tags association` do
+      it 'does not return the tags association' do
         expect(service_model.ar_model_associations).to_not include(:tags)
       end
 
-      it `does not include associations from superclass` do
+      it 'does not include associations from superclass' do
         expect(service_model.ar_model_associations).to_not include(service_model.superclass.ar_model_associations)
       end
     end