Used feature.peak_local_max instead of deprecated function

Hello,

I've been using and sharing documentation; I stumbled across http://scikit-image.org/docs/dev/auto_examples/plot_watershed.html and thought it would be nicer that the example be DeprecationWarning-free. :)

I contemplated strict PEP8 compliance (lines 38--39) but... We want to keep writing x**n for 'x to the power of n', right?