Fix URL in comment around origin of matplotlib drawer (#1051)

The rustworkx mpl_draw() function was originally adapted from networkx's
matplotlib based drawer and heavily modified to work with rustworkx and
also conform more to rustworkx's interface style. Attribution to the
original source and the license terms of the original networkx code are
present in a comment at a top of the module. This port was originally
done when rustworkx was still called retworkx. As part of the name
migration in #644 an overzealous find/replace broke the URL to the
permalink to the original networkx source code that the mpl_draw()
function was based on. This commit corrects this oversight so that the
link correctly resolves to the proper url.

Co-authored-by: Ivan Carvalho <[email protected]>

rustworkx/visualization/matplotlib.py

@@ -45,7 +45,7 @@
 # This code is forked from networkx's networkx_pylab.py module and adapted to
 # work with rustworkx instead. The original source can be found at:
 #
-# https://github.com/rustworkx/rustworkx/blob/80b1afa2ae50314a8312998c214a8c1a356adcf1/rustworkx/drawing/rustworkx_pylab.py
+# https://github.com/networkx/networkx/blob/80b1afa2ae50314a8312998c214a8c1a356adcf1/networkx/drawing/nx_pylab.py
 
 """Draw a rustworkx graph with matplotlib."""