bpf, docs: Fix broken link to renamed bpf_iter_task_vmas.c

This file was renamed from bpf_iter_task_vma.c.

Fixes: 45b38941 ("selftests/bpf: Rename bpf_iter_task_vma.c to bpf_iter_task_vmas.c")
Signed-off-by: T.J. Mercier <tjmercier@google.com>
Acked-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20250304204520.201115-1-tjmercier@google.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>