Updates Slack integration helptext copy

app/views/projects/services/slack/_help.haml

@@ -3,7 +3,7 @@
 
 .info-well
   .well-segment
-    %p= s_('SlackIntegration|This service send notifications about projects\' events to Slack channels. To set up this service:')
+    %p= s_('SlackIntegration|This service sends notifications about project events to Slack channels. To set up this service:')
     %ol
       %li
         = html_escape(s_('SlackIntegration|%{webhooks_link_start}Add an incoming webhook%{webhooks_link_end} in your Slack team. The default channel can be overridden for each event.')) % { webhooks_link_start: webhooks_link_start.html_safe, webhooks_link_end: '</a>'.html_safe }

locale/gitlab.pot

@@ -22471,7 +22471,7 @@ msgstr ""
 msgid "SlackIntegration|Select events below to enable notifications. The %{strong_open}Slack channel names%{strong_close} and %{strong_open}Slack username%{strong_close} fields are optional."
 msgstr ""
 
-msgid "SlackIntegration|This service send notifications about projects' events to Slack channels. To set up this service:"
+msgid "SlackIntegration|This service sends notifications about project events to Slack channels. To set up this service:"
 msgstr ""
 
 msgid "SlackService|2. Paste the %{strong_open}Token%{strong_close} into the field below"