Fix number of thread in javadoc

[nagetsum]

According to the below code, default maxThreads is 8 * the number of available processors.

https://github.com/quarkusio/quarkus/blob/master/core/runtime/src/main/java/io/quarkus/runtime/ExecutorTemplate.java#L168

builder.setMaximumPoolSize(threadPoolConfig.maxThreads.orElse(8 * cpus));