tracing/histogram: Update document for KEYS_MAX size

commit a635beea upstream.

After commit 4f36c2d8

 ("tracing: Increase tracing map KEYS_MAX size"),
'keys' supports up to three fields.

Signed-off-by: Zheng Yejian <zhengyejian1@huawei.com>
Cc: stable@vger.kernel.org
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Link: https://lore.kernel.org/r/20221017103806.2479139-1-zhengyejian1@huawei.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>