Fix docstring.

wrappers/python/simtk/openmm/app/amberprmtopfile.py

@@ -203,10 +203,10 @@ class AmberPrmtopFile(object):
         ewaldErrorTolerance : float=0.0005
             The error tolerance to use if nonbondedMethod is Ewald or PME.
         switchDistance : float=0*nanometers
-            A positive value turns on a potential energy switching function for
-            Lennard-Jones interactions. If the switchDistance is 0 or evaluates
-            to boolean False, no switching function will be used. Illegal values
-            (e.g., less than 0) will raise a ValueError
+            The distance at which the potential energy switching function is
+            turned on. If the switchDistance is 0 or evaluates to boolean False,
+            no switching function will be used. Values greater than
+            nonbondedCutoff or less than 0 raise a ValueError
 
         Returns
         -------