tools: bpftool: Fix formatting in bpftool-link documentation

Fix a formatting error in the documentation for bpftool-link, so that
the man page can build correctly.

Signed-off-by: Quentin Monnet <quentin@isovalent.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200904161454.31135-2-quentin@isovalent.com