fix label for schedule publish invariant content

src/Umbraco.Web.UI.Client/src/views/content/overlays/schedule.html

@@ -4,7 +4,7 @@
     <div ng-if="vm.variants.length === 1">
 
         <div style="margin-bottom: 15px;">
-            <p><localize key="content_languagesToSchedule"></localize></p>
+            <p><localize key="content_schedulePublishHelp"></localize></p>
         </div>
 
         <div class="date-wrapper">

src/Umbraco.Web.UI/Umbraco/config/lang/en_us.xml

@@ -264,6 +264,7 @@
     <key alias="readyToPublish">Ready to Publish?</key>
     <key alias="readyToSave">Ready to Save?</key>
     <key alias="sendForApproval">Send for approval</key>
+    <key alias="schedulePublishHelp">Select the date and time to publish and/or unpublish the content item.</key>
   </area>
   <area alias="blueprints">
     <key alias="createBlueprintFrom">Create a new Content Template from '%0%'</key>