Update Makefile comment to indicate arm64 as a supported Linux desktop platform (#2965)

src/Makefile

@@ -4,7 +4,7 @@
 #
 #  Platforms supported:
 #    PLATFORM_DESKTOP:  Windows (Win32, Win64)
-#    PLATFORM_DESKTOP:  Linux (i386, x64)
+#    PLATFORM_DESKTOP:  Linux (arm64, i386, x64)
 #    PLATFORM_DESKTOP:  OSX/macOS (arm64, x86_64)
 #    PLATFORM_DESKTOP:  FreeBSD, OpenBSD, NetBSD, DragonFly
 #    PLATFORM_ANDROID:  Android (arm, i686, arm64, x86_64)