bpftool: fix some typos in bpftool

Hi, fix some spelling errors in bpftool, the details are as follows:

-in file "bpftool-gen.rst"
	libppf->libbpf
-in the code comments:
	ouptut->output

Signed-off-by: Lin Yikai <yikai.lin@vivo.com>
Link: https://lore.kernel.org/r/20240905110354.3274546-2-yikai.lin@vivo.com


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