libbpf, doc: Eliminate warnings in libbpf_naming_convention

Use "code-block: none" instead of "c" for non-C-language code blocks.
Removes these warnings:

  lnx-514-rc4/Documentation/bpf/libbpf/libbpf_naming_convention.rst:111: WARNING: Could not lex literal_block as "c". Highlighting skipped.
  lnx-514-rc4/Documentation/bpf/libbpf/libbpf_naming_convention.rst:124: WARNING: Could not lex literal_block as "c". Highlighting skipped.

Fixes: f42cfb46 ("bpf: Add documentation for libbpf including API autogen")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20210802015037.787-1-rdunlap@infradead.org