Revise assignee section of page for tone / style

The Assignee section of the alert management page needed some tweaks
to adhere to GitLab tone and style.

doc/user/project/operations/alert_management.md

+---
+stage: Monitor
+group: Health
+info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#designated-technical-writers
+---
+
 # Alert Management
 
 > [Introduced](https://gitlab.com/groups/gitlab-org/-/epics/2877) in GitLab 13.0.
@@ -95,16 +101,13 @@ See [Alert Management statuses](#alert-management-statuses) for more details abo
 
 ### Update an Alert's assignee
 
-NOTE: **Note:**
-We currently only support a single assignee per alert.
-
-The Alert Management detail view allows users to update the Alert Assignee(s).
-
 > [Introduced](https://gitlab.com/groups/gitlab-org/-/epics/3066) in GitLab 13.1.
 
+The Alert Management detail view allows users to update the Alert assignee.
+
 In large teams, where there is shared ownership of an alert, it can be difficult
 to track who is investigating and working on it. The Alert Management detail view
-enables you to update the Alert Assignee(s):
+enables you to update the Alert assignee:
 
 NOTE: **Note:**
 GitLab currently only supports a single assignee per alert.