docs: bpf: Capitalise document heading

The majority of files in the kernel documentation index use
capitalisation for all words, especially the shorter ones.  BPF docs
better fit in with the rest of the documentation if the heading is all
capitalised.

Capitalise document heading.

Signed-off-by: Tobin C. Harding <me@tobin.cc>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>