Fix T71508: wrong gravity settings in scene defaults

Typo in rBf5e0dfe59c7e.
Showed when creating a new default scene.

Maniphest Tasks: T71508

Differential Revision: https://developer.blender.org/D6229

source/blender/makesdna/DNA_scene_defaults.h

@@ -171,7 +171,7 @@
 
 #define _DNA_DEFAULT_PhysicsSettings \
   { \
-    .gravity = {0.0f, 0.0f -9.81f}, \
+    .gravity = {0.0f, 0.0f, -9.81f}, \
     .flag = PHYS_GLOBAL_GRAVITY, \
   }