Change PhysicsMaterial extension to phymat

All other binary extensions are lowercase.

scene/resources/physics_material.h

@@ -37,7 +37,7 @@ class PhysicsMaterial : public Resource {
 
 	GDCLASS(PhysicsMaterial, Resource);
 	OBJ_SAVE_TYPE(PhysicsMaterial);
-	RES_BASE_EXTENSION("PhyMat");
+	RES_BASE_EXTENSION("phymat");
 
 	real_t friction;
 	bool rough;