Copy edit AT_CellularStack.h

Copy edit for American English.

Author: Amanda Butler

features/cellular/framework/AT/AT_CellularStack.h

@@ -88,7 +88,7 @@ class AT_CellularStack : public NetworkStack, public AT_CellularBase
         void (*_cb)(void *);
         void *_data;
         bool created;
-        bool rx_avail; // used to synchronise reading from modem
+        bool rx_avail; // used to synchronize reading from modem
     };
 
     /**