Fix grammar in shards availability health indicator diagnosis

server/src/main/java/org/elasticsearch/cluster/routing/allocation/shards/ShardsAvailabilityHealthIndicatorService.java

@@ -254,7 +254,7 @@ public class ShardsAvailabilityHealthIndicatorService implements HealthIndicator
         NAME,
         "initializing_shards",
         "Elasticsearch is currently initializing the unavailable shards. Please wait for the initialization to finish.",
-        "The shards will become available as long as the initialization completes. No action is required by the user, you can"
+        "The shards will become available when the initialization completes. No action is required by the user, you can"
             + " monitor the progress of the initializing shards at "
             + WAIT_FOR_INITIALIZATION_GUIDE
             + ".",