docs, bpf: Add minimal markup to address doc warning

Commit 91c960b ("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: 91c960b ("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

Author: bulwahn

Documentation/networking/filter.rst

@@ -1048,12 +1048,12 @@ Unlike classic BPF instruction set, eBPF has generic load/store operations::
 Where size is one of: BPF_B or BPF_H or BPF_W or BPF_DW.
 
 It also includes atomic operations, which use the immediate field for extra
-encoding.
+encoding::
 
    .imm = BPF_ADD, .code = BPF_ATOMIC | BPF_W  | BPF_STX: lock xadd *(u32 *)(dst_reg + off16) += src_reg
    .imm = BPF_ADD, .code = BPF_ATOMIC | BPF_DW | BPF_STX: lock xadd *(u64 *)(dst_reg + off16) += src_reg
 
-The basic atomic operations supported are:
+The basic atomic operations supported are::
 
     BPF_ADD
     BPF_AND