Fixed cross product formula

Components should be multiplied when calculating the cross product, not added.
(cherry picked from commit dccd2ac732)

tutorials/vector_math.rst

@@ -676,9 +676,9 @@ The formula for the cross product is:
 ::
 
     var c = Vector3()
-    c.x = (a.y + b.z) - (a.z + b.y)
-    c.y = (a.z + b.x) - (a.x + b.z)
-    c.z = (a.x + b.y) - (a.y + b.x)
+    c.x = (a.y * b.z) - (a.z * b.y)
+    c.y = (a.z * b.x) - (a.x * b.z)
+    c.z = (a.x * b.y) - (a.y * b.x)
 
 This can be simplified, in Godot, to: