docs, bpf: Add minimal markup to address doc warning

Commit 91c960b0 ("bpf: Rename BPF_XADD and prepare to encode other
atomics in .imm") modified the BPF documentation, but missed some ReST
markup.

Hence, make htmldocs warns on Documentation/networking/filter.rst:1053:

  WARNING: Inline emphasis start-string without end-string.

Add some minimal markup to address this warning.

Fixes: 91c960b0 ("bpf: Rename BPF_XADD and prepare to encode other atomics in .imm")
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Brendan Jackman <jackmanb@google.com>
Link: https://lore.kernel.org/bpf/20210118080004.6367-1-lukas.bulwahn@gmail.com


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