Update data-collector-api.md

-------
cc: @bwren

Author: bwren

articles/azure-monitor/logs/data-collector-api.md

@@ -364,6 +364,7 @@ namespace OIAPIExample
 				client.DefaultRequestHeaders.Add("x-ms-date", date);
 				client.DefaultRequestHeaders.Add("time-generated-field", TimeStampField);
 
+				// If charset=utf-8 is part of the content-type header, the API call may return forbidden.
 				System.Net.Http.HttpContent httpContent = new StringContent(json, Encoding.UTF8);
 				httpContent.Headers.ContentType = new MediaTypeHeaderValue("application/json");
 				Task<System.Net.Http.HttpResponseMessage> response = client.PostAsync(new Uri(url), httpContent);