Update 4.8DiffTraj.md

docs/user_guide/4.8DiffTraj.md

@@ -1,6 +1,6 @@
 # DiffTraj
 ## 1. Theory
-DiffTraj provides a support to gradient calculation on a loss function from trajectory. First a NVE (leap-frog Verlet) simulation is conducted, and then compute the gradient from the trajectory.
+DiffTraj provides a support to do gradient calculation on a loss function from trajectory. First a NVE (leap-frog Verlet) simulation is conducted, and then compute the gradient from the trajectory.
 ### 1.1 NVE simulation
 NVE simulation follows the leap-frog Verlet integration method, just like in openMM. The positions and velocities stored in the context are offset from each other by half a time step. In each step, they are updated as follows: