ioctl: Fix broken link to the kernel documentation
The file documenting ioctl number assignment in the Linux kernel has been converted to ReStructuredText (see for some background), the file extension has naturally changed.
//! Historically `ioctl` numbers were arbitrary hard-coded values. In Linux (before 2.6) and some
//! unices this has changed to a more-ordered system where the ioctl numbers are partitioned into
//! subcomponents (For linux this is documented in
-//! [`Documentation/ioctl/ioctl-number.txt`](http://elixir.free-electrons.com/linux/latest/source/Documentation/ioctl/ioctl-number.txt)):
+//! [`Documentation/ioctl/ioctl-number.rst`](http://elixir.free-electrons.com/linux/latest/source/Documentation/ioctl/ioctl-number.rst)):
//!
//! * Number: The actual ioctl ID
//! * Type: A grouping of ioctls for a common purpose or driver
