bpf, docs: Fix literal block for example code

Add a missing colon so that the code block followed can be rendered
properly.
Signed-off-by: Hengqi Chen <hengqi.chen@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20210424021208.832116-1-hengqi.chen@gmail.com

Documentation/networking/filter.rst

@@ -327,7 +327,7 @@ Examples for low-level BPF:
   ret #-1
   drop: ret #0
 
-**icmp random packet sampling, 1 in 4**:
+**icmp random packet sampling, 1 in 4**::
 
   ldh [12]
   jne #0x800, drop