doc: remove wrong limitation in RCU guide
doc: remove wrong limitation in RCU guide

There is no limitation of 1024 reader threads.

Fixes: 64994b56cfd7 ("rcu: add RCU library supporting QSBR mechanism")
Cc: stable@dpdk.org

Signed-off-by: Honnappa Nagarahalli <01ccf438b971cc1d8919c8a7f18443cae9429d8b@arm.com>
Reviewed-by: Ruifeng Wang <a548d51fbc42485a8ba807711a537e88c2b59dd2@arm.com>
Reviewed-by: Gavin Hu <207ee22c2cf71e20c5b5df8395050c07c07d8709@arm.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk