bpf: Fix string kfuncs names in doc comments

bpf: Fix string kfuncs names in doc comments

Documentation comments for bpf_strnlen and bpf_strcspn contained
incorrect function names.

Fixes: e91370550f1f ("bpf: Add kfuncs for read-only string operations")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Closes: https://lore.kernel.org/bpf/20250627174759.3a435f86@canb.auug.org.au/T/#u
Signed-off-by: Viktor Malik <vmalik@redhat.com>
Link: https://lore.kernel.org/r/20250627082001.237606-1-vmalik@redhat.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>