correct docstring for displacement dynamic

Author: piotrbartman

PySDM/dynamics/displacement.py

@@ -3,7 +3,7 @@
 
 adaptive time-stepping controlled by comparing implicit-Euler (I)
 and explicit-Euler (E) maximal displacements with:
-rtol < |(I - E) / E|
+rtol > |(I - E) / E|
 (see eqs 13-16 in [Arabas et al. 2015](https://doi.org/10.5194/gmd-8-1677-2015))
 """
 from collections import namedtuple