rust: ioctl: end top-level module docs with full stop

Every other module ends its first line of documentation with a full
stop. Adapt the only outlier.
Signed-off-by: Valentin Obst <kernel@valentinobst.de>
Reviewed-by: Trevor Gross <tmgross@umich.edu>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20240131-doc-fixes-v3-v3-3-0c8af94ed7de@valentinobst.deSigned-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/kernel/ioctl.rs

 // SPDX-License-Identifier: GPL-2.0
 
-//! ioctl() number definitions
+//! ioctl() number definitions.
 //!
 //! C header: [`include/asm-generic/ioctl.h`](srctree/include/asm-generic/ioctl.h)