docs: Fix docstring for strain increment calculation

src/pydrex/utils.py

@@ -15,7 +15,7 @@
 def strain_increment(dt, velocity_gradient):
     """Calculate strain increment for a given time increment and velocity gradient.
 
-    Returns “tensorial” strain increment ε, which is equal to 2 × γ where γ is the
+    Returns “tensorial” strain increment ε, which is equal to γ/2 where γ is the
     “(engineering) shear strain” increment.
 
     """