[multibody] Fix whitespace (RobotLocomotion#21312)

Author: rpoyner-tri

multibody/tree/quaternion_floating_mobilizer.h

@@ -23,7 +23,7 @@ namespace internal {
 // introduces four generalized positions to describe the orientation R_FM of
 // frame M in F with a quaternion q_FM, and three generalized positions to
 // describe the translation of frame M's origin in F with a position vector
-// p_FM. The seven entries of the configuration vector q are ordered 
+// p_FM. The seven entries of the configuration vector q are ordered
 // (q_FM, p_FM) with the quaternion, ordered wxyz (scalar then vector),
 // preceding the translation vector. As generalized velocities, this mobilizer
 // introduces the angular velocity w_FM of frame M in F and the linear