docs, kprobes: Add loongarch as supported architecture

After the following three changes at the beginning of the year:

  commit 6d4cc40f ("LoongArch: Add kprobes support")
  commit 3f553686 ("LoongArch: Add kretprobes support")
  commit 09e679c2

 ("LoongArch: Add kprobes on ftrace support")

it is appropriate to add loongarch as supported architecture in
kprobes documentation.

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Acked-by: "Masami Hiramatsu (Google)" <mhiramat@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20231219062330.22813-3-yangtiezhu@loongson.cn