Fixed typo 'thead' in thread_neighbor_class.h

Note that the word 'thead' is wrong,
so that 'thead' should been replaced with thread'.

Author: DavidLin1577

features/nanostack/sal-stack-nanostack/source/6LoWPAN/Thread/thread_neighbor_class.h

@@ -32,7 +32,7 @@
 
 struct thread_neighbor_class_s;
 
-/** Thead Spesific ModeFlags */
+/** Thread Spesific ModeFlags */
 #define MLE_THREAD_SECURED_DATA_REQUEST 0x04
 #define MLE_THREAD_REQ_FULL_DATA_SET 0x01