[ML] Capitalize first letter in error message

Original commit: elastic/x-pack-elasticsearch@7062851357

plugin/core/src/main/java/org/elasticsearch/xpack/ml/action/PostCalendarEventsAction.java

@@ -84,7 +84,7 @@ public class PostCalendarEventsAction extends Action<PostCalendarEventsAction.Re
             this.scheduledEvents = ExceptionsHelper.requireNonNull(scheduledEvents, EVENTS.getPreferredName());
 
             if (scheduledEvents.isEmpty()) {
-                throw ExceptionsHelper.badRequestException("at least 1 event is required");
+                throw ExceptionsHelper.badRequestException("At least 1 event is required");
             }
         }
 

plugin/src/test/resources/rest-api-spec/test/ml/calendar_crud.yml

@@ -596,7 +596,7 @@
 "Test post calendar events given empty events":
 
   - do:
-      catch: /at least 1 event is required/
+      catch: /At least 1 event is required/
       xpack.ml.post_calendar_events:
         calendar_id: "foo"
         body: >