update guide

Author: bradjc

doc/guide.md

@@ -251,12 +251,12 @@ typedef void (*libtock_sensor_callback_reading)(returncode_t, int);
 Define a upcall function to be passed to the kernel:
 
 ```c
-static void sensor_temp_upcall(int                          ret,
+static void sensor_temp_upcall(int                          status,
                                int                          val,
                                __attribute__ ((unused)) int unused0,
                                void*                        opaque) {
   libtock_sensor_callback_reading cb = (libtock_sensor_callback_reading) opaque;
-  cb(ret, val);
+  cb(tock_status_to_returncode(ret), val);
 }
 ```