Fix typo in McpSession.cs (#371)

Author: stephentoub

src/ModelContextProtocol/Shared/McpSession.cs

@@ -753,7 +753,7 @@ private static TimeSpan GetElapsed(long startingTimestamp) =>
     [LoggerMessage(Level = LogLevel.Warning, Message = "{EndpointName} received unexpected {MessageType} message type.")]
     private partial void LogEndpointHandlerUnexpectedMessageType(string endpointName, string messageType);
 
-    [LoggerMessage(Level = LogLevel.Warning, Message = "{EndpointName} received request for method '{Method}', but not handler is available.")]
+    [LoggerMessage(Level = LogLevel.Warning, Message = "{EndpointName} received request for method '{Method}', but no handler is available.")]
     private partial void LogNoHandlerFoundForRequest(string endpointName, string method);
 
     [LoggerMessage(Level = LogLevel.Debug, Message = "{EndpointName} waiting for response to request '{RequestId}' for method '{Method}'.")]
@@ -764,4 +764,4 @@ private static TimeSpan GetElapsed(long startingTimestamp) =>
 
     [LoggerMessage(Level = LogLevel.Trace, Message = "{EndpointName} sending message. Message: '{Message}'.")]
     private partial void LogSendingMessageSensitive(string endpointName, string message);
-}
+}