Update plotting_backends.md (#2364)

Change path of function provided in documentation to right path

doc/source/contributing/plotting_backends.md

@@ -13,7 +13,7 @@ code in `arviz.plots.backends` perform the backend specific
 keyword argument defaulting and plot behavior.
 
 The convenience function `get_plotting_function` available in
-`arviz.plots.get_plotting_function` should be called to obtain
+`arviz.plots.plot_utils.get_plotting_function` should be called to obtain
 the correct plotting function from the associated backend. If
 adding a new backend follow the pattern provided to programmatically
 call the correct backend.