Remove reference to optimize from cahnhilliard doc

This patch removes the reference to solver.optimize from the docstring of the
cahnhilliard example, which switched to using System.solve instead.

examples/cahnhilliard.py

@@ -114,7 +114,7 @@ def main(size: Length = parse('10cm'),
 
     For this reason, the `stab` enum in this script defines the stabilizing term
     `δψ`, rather than `f`, allowing Nutils to construct the residuals through
-    automatic differentiation using the `optimize` method.
+    symbolic differentiation.
 
     Parameters
     ----------