bpftool: Fix mixed indentation in documentation

Some paragraphs in bpftool's documentation have a mix of tabs and spaces
for indentation. Let's make it consistent.

This patch brings no change to the text content.

Signed-off-by: Quentin Monnet <quentin@isovalent.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20211110114632.24537-7-quentin@isovalent.com