adding get actor reminder API in docs (dapr#1113)

Signed-off-by: Shivam Kumar <shivamkm07@gmail.com>

daprdocs/content/en/dotnet-sdk-docs/dotnet-actors/dotnet-actors-howto.md

@@ -81,6 +81,7 @@ Define `IMyActor` interface and `MyData` data object. Paste the following code i
 
 ```csharp
 using Dapr.Actors;
+using Dapr.Actors.Runtime;
 using System.Threading.Tasks;
 
 namespace MyActor.Interfaces
@@ -91,6 +92,7 @@ namespace MyActor.Interfaces
         Task<MyData> GetDataAsync();
         Task RegisterReminder();
         Task UnregisterReminder();
+        Task<IActorReminder> GetReminder();
         Task RegisterTimer();
         Task UnregisterTimer();
     }
@@ -219,6 +221,14 @@ namespace MyActorService
                 TimeSpan.FromSeconds(5));  // Time interval between reminder invocations after the first invocation
         }
 
+        /// <summary>
+        /// Get MyReminder reminder details with the actor
+        /// </summary>
+        public async Task<IActorReminder> GetReminder()
+        {
+            await this.GetReminderAsync("MyReminder");
+        }
+
         /// <summary>
         /// Unregister MyReminder reminder with the actor
         /// </summary>