bpf: Correct the comment for BTF kind bitfield

The commit 8fd88691 ("bpf: Add BTF_KIND_FLOAT to uapi") has extended
the BTF kind bitfield from 4 to 5 bits, correct the comment.

Signed-off-by: Haiyue Wang <haiyue.wang@intel.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220403115327.205964-1-haiyue.wang@intel.com