docs: update config.yaml.example about upstream's keepalive. #689

Job	Run time
build (ubuntu-20.04, linux_openresty, lua-resty-worker-events, t/http3/admin)	17m 27s
build (ubuntu-20.04, linux_openresty, lua-resty-worker-events, t/plugin/[a-k]*)	33m 55s
build (ubuntu-20.04, linux_openresty, lua-resty-worker-events, t/plugin/[l-z]*)	33m 57s
build (ubuntu-20.04, linux_openresty, lua-resty-worker-events, t/admin t/cli t/config-center-yaml...	24m 22s
build (ubuntu-20.04, linux_openresty, lua-resty-worker-events, t/node t/pubsub t/router t/script ...	25m 22s
build (ubuntu-20.04, linux_openresty, lua-resty-events, t/http3/admin)	15m 17s
build (ubuntu-20.04, linux_openresty, lua-resty-events, t/plugin/[a-k]*)	33m 40s
build (ubuntu-20.04, linux_openresty, lua-resty-events, t/plugin/[l-z]*)	36m 1s
build (ubuntu-20.04, linux_openresty, lua-resty-events, t/admin t/cli t/config-center-yaml t/cont...	26m 55s
build (ubuntu-20.04, linux_openresty, lua-resty-events, t/node t/pubsub t/router t/script t/secre...	26m 7s
	4h 33m 3s

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