Update copy to show multiple channels

app/models/project_services/slack_service.rb

@@ -19,12 +19,12 @@ class SlackService < ChatNotificationService
     <ol>
       <li><a href="https://slack.com/apps/A0F7XDUAZ-incoming-webhooks">Add an incoming webhook</a> in your Slack team. The default channel can be overridden for each event.</li>
       <li>Paste the <strong>Webhook URL</strong> into the field below.</li>
-      <li>Select events below to enable notifications. The <strong>Channel name</strong> and <strong>Username</strong> fields are optional.</li>
+      <li>Select events below to enable notifications. The <strong>Channel names</strong> and <strong>Username</strong> fields are optional.</li>
     </ol>'
   end
 
   def default_channel_placeholder
-    "Channel name (e.g. general)"
+    _('Channel names (e.g. general, people-ops)')
   end
 
   def webhook_placeholder

locale/gitlab.pot

@@ -3336,6 +3336,9 @@ msgstr ""
 msgid "Changing group path can have unintended side effects."
 msgstr ""
 
+msgid "Channel names (e.g. general, people-ops)"
+msgstr ""
+
 msgid "Charts"
 msgstr ""