Updating example code to current version

The stringify versinofunction to verbose the content of expressions or PrimitiveTrees is no longer supported and the str function has to be used instead.

doc/tutorials/advanced/gp.rst

@@ -175,13 +175,13 @@ Evaluation of Trees
 
 In DEAP, trees can be translated to readable Python code and compiled to Python
 code objects using functions provided by the :py:mod:`~deap.gp` module. The first
-function, :func:`~deap.gp.stringify` takes an expression or a PrimitiveTree and
+function, :func:`str` takes an expression or a PrimitiveTree and
 translates it into readable Python code. For example, the following lines
 generate a tree and output the code from the first example primitive set. ::
 
 	>>> expr = genFull(pset, min_=1, max_=3)
 	>>> tree = PrimitiveTree(expr)
-	>>> stringify(tree)
+	>>> str(tree)
 	'mul(add(x, x), max(y, x))'
 
 Now, this string represents the program we just generated, but it cannot yet be