Documentation: English corrections in vmalloced kernel stacks

Minor grammar fixes in vmalloced-kernel-stacks

Signed-off-by: Nir Lichtman <nir@lichtman.org>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240619215944.GA3571421@lichtman.org