Correct default thread pool size in README.md

[falbrechtskirchinger]

cpp-httplib/httplib.h

Lines 129 to 134 in a4b2c61

	#ifndef CPPHTTPLIB_THREAD_POOL_COUNT
	#define CPPHTTPLIB_THREAD_POOL_COUNT \
	((std::max)(8u, std::thread::hardware_concurrency() > 0 \
	? std::thread::hardware_concurrency() - 1 \
	: 0))
	#endif

[yhirose]

Thanks for the correction!