Fix macOS Print Screen

Many years ago this value was changed from F13 to Print Screen with
1ae9ce1e0a00bf6f8b7e719bf1ec6e73f111afcf.  However, this was later
reverted to F13 with 5759d0fdf272351b35f666718a6e2a665db827e8.

This changes it back to Print Screen for consistency with other
platforms.

Closes #1786
Fixes #2169

(cherry picked from commit c940695eb03723fa6e4b174db524bcaa4f55bbee)

src/cocoa_init.m

@@ -251,7 +251,7 @@ static void createKeyTables(void)
     _glfw.ns.keycodes[0x6D] = GLFW_KEY_F10;
     _glfw.ns.keycodes[0x67] = GLFW_KEY_F11;
     _glfw.ns.keycodes[0x6F] = GLFW_KEY_F12;
-    _glfw.ns.keycodes[0x69] = GLFW_KEY_F13;
+    _glfw.ns.keycodes[0x69] = GLFW_KEY_PRINT_SCREEN;
     _glfw.ns.keycodes[0x6B] = GLFW_KEY_F14;
     _glfw.ns.keycodes[0x71] = GLFW_KEY_F15;
     _glfw.ns.keycodes[0x6A] = GLFW_KEY_F16;