Fix healthcheck config example (#16)

config/sys.config

@@ -26,7 +26,7 @@
             idle_timeout => infinity
         }},
         {shutdown_timeout, 7000},
-        {health_checkers, #{
+        {health_check, #{
             disk    => {erl_health, disk     , ["/", 99]        },
             memory  => {erl_health, cg_memory, [99]             },
             service => {erl_health, service  , [<<"sequences">>]}