Minor README.md update that was missing from #14981. NFC

Author: sbc100

system/lib/README.md

@@ -17,7 +17,7 @@ These current set of static constructors in system libraries and their prioritie
 (lowest run first) are:
 
 - 47: `initialize_emmalloc_heap` (emmalloc.c)
-- 48: `__emscripten_pthread_data_constructor` (pthread/library_pthread.c)
+- 48: `__emscripten_init_main_thread` (pthread/library_pthread.c)
 - 50: asan init (??)
 
 Priorities 0 - 100 are reserved for system libraries and user-level