Use Apple's nomenclature for macOS and iOS

Fixes https://github.com/libsdl-org/SDL/issues/6621

src/timer/unix/SDL_systimer.c

@@ -43,7 +43,7 @@
 /* Reworked monotonic clock to not assume the current system has one
    as not all linux kernels provide a monotonic clock (yeah recent ones
    probably do)
-   Also added OS X Monotonic clock support
+   Also added macOS Monotonic clock support
    Based on work in https://github.com/ThomasHabets/monotonic_clock
  */
 #if HAVE_NANOSLEEP || HAVE_CLOCK_GETTIME