docs: bpf: Fix bullet point formatting warning

Fix indentation for a bullet list item in bpf_iterators.rst.
According to reStructuredText rules, bullet list item bodies must be
consistently indented relative to the bullet. The indentation of the
first line after the bullet determines the alignment for the rest of
the item body.

Reported by smatch:
  /linux/Documentation/bpf/bpf_iterators.rst:55: WARNING: Bullet list ends without a blank line; unexpected unindent. [docutils]

Fixes: 7220eabf ("bpf, docs: document open-coded BPF iterators")
Signed-off-by: Khaled Elnaggar <khaledelnaggarlinux@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20250513015901.475207-1-khaledelnaggarlinux@gmail.com