Rename RigidBodyBuilder::principal_inertia -> principal_angular_inertia for clarity.

examples3d/debug_add_remove_collider3.rs

@@ -1,4 +1,4 @@
-use na::{Isometry3, Point3, Vector3};
+use na::Point3;
 use rapier3d::dynamics::{JointSet, RigidBodyBuilder, RigidBodySet};
 use rapier3d::geometry::{ColliderBuilder, ColliderSet};
 use rapier_testbed3d::Testbed;