`spiral_varden()` docstring appears to have wrong units for `gslew` argument

[tomdstone]

Describe the bug
In the documentation for the sigpy.mri.rf.trajgrad.spiral_varden() function, it says that argument gslew takes in units of g/cm/s. Which is equal to mT/m/s. On the MRI machine I'm simulating, my value is 750 mT/m/ms, so I have to provide it as 750e3 to get the units right. But that completely messes up the trajectory, while gslew = 750 itself creates the spiral as I expected it to be. That makes me think that the docs should say g/cm/ms