[Guided onboarding] Copy update for the Security rules step (elastic#145887)

## Summary
This PR adds a sentence to the Rules step in the Security guide. 

<img width="458" alt="Screenshot 2022-11-21 at 17 04 01"
src="https://user-images.githubusercontent.com/6585477/203102051-d3fe053b-4cf1-499d-937d-64acda7c7c41.png">

cc @paulewing

Author: yuliacech

src/plugins/guided_onboarding/public/constants/guides_config/security.ts

@@ -53,6 +53,9 @@ export const securityConfig: GuideConfig = {
         i18n.translate('guidedOnboarding.securityGuide.rulesStep.description2', {
           defaultMessage: 'Select the rules that you want.',
         }),
+        i18n.translate('guidedOnboarding.securityGuide.rulesStep.description3', {
+          defaultMessage: 'Enable rules to generate alerts.',
+        }),
       ],
       manualCompletion: {
         title: i18n.translate('guidedOnboarding.securityGuide.rulesStep.manualCompletion.title', {