Documentation: siphash: Fix typo in the name of offsetofend macro

The siphash documentation misspelled "offsetendof" instead of
"offsetofend".

Fixes: 2c956a60 ("siphash: add cryptographically secure PRF")
Signed-off-by: Dov Murik <dovmurik@linux.ibm.com>
Acked-by: Jason A. Donenfeld <Jason@zx2c4.com>
Link: https://lore.kernel.org/r/20220712104455.1408150-1-dovmurik@linux.ibm.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>