minor fix.

Author: rezasherafat

articles/iot-hub/iot-hub-devguide-device-twins.md

@@ -240,7 +240,7 @@ The [Azure IoT device SDKs](iot-hub-devguide-sdks.md) make it easy to use the pr
 
 Tags, desired properties, and reported properties are JSON objects with the following restrictions:
 
-* All keys in JSON objects are UTF-8 encoded, case-sensitive, and up-to 64 bytes in length. Allowed characters exclude UNICODE control characters (segments C0 and C1), and `.`, `$`, and SP.
+* All keys in JSON objects are UTF-8 encoded, case-sensitive, and up-to 1 KB in length. Allowed characters exclude UNICODE control characters (segments C0 and C1), and `.`, `$`, and SP.
 
 * All values in JSON objects can be of the following JSON types: boolean, number, string, object. Arrays are not allowed. The maximum value for integers is 4503599627370495 and the minimum value for integers is -4503599627370496.