Fixed a typo error in a comment.

src/modbus.c

@@ -1467,7 +1467,7 @@ int modbus_mask_write_register(modbus_t *ctx, int addr, uint16_t and_mask, uint1
     int rc;
     int req_length;
     /* The request length can not exceed _MIN_REQ_LENGTH - 2 and 4 bytes to
-     * store the masks. The ugly subtraction is there to remove the 'nb' value
+     * store the masks. The ugly substraction is there to remove the 'nb' value
      * (2 bytes) which is not used. */
     uint8_t req[_MIN_REQ_LENGTH + 2];