Rename file used for non-existent file spec

Author: cacheflow

spec/fcm_spec.rb

@@ -90,7 +90,7 @@
     end
 
     it 'raises an error when passed a string of a file that does not exist' do
-      fcm = FCM.new('fake_credentials.json', '', {})
+      fcm = FCM.new('example.txt', '', {})
       expect { fcm.__send__(:json_key) }.to raise_error(creds_error)
     end
   end