Update IAsyncStreamWriter.cs

Author: JamesNK

src/csharp/Grpc.Core.Api/IAsyncStreamWriter.cs

@@ -31,9 +31,9 @@ namespace Grpc.Core
     public interface IAsyncStreamWriter<in T>
     {
         /// <summary>
-        /// Writes a single asynchronously. Only one write can be pending at a time.
+        /// Writes a message asynchronously. Only one write can be pending at a time.
         /// </summary>
-        /// <param name="message">the message to be written. Cannot be null.</param>
+        /// <param name="message">The message to be written. Cannot be null.</param>
         Task WriteAsync(T message);
 
         /// <summary>