wp example missing a minus sign when defining bin centers

[f-ardila]

In Galaxy Catalog Analysis Example: Projected galaxy clustering
(https://halotools.readthedocs.io/en/latest/quickstart_and_tutorials/tutorials/catalog_analysis/galcat_analysis/basic_examples/clustering_examples/galaxy_catalog_analysis_tutorial4.html#galaxy-catalog-analysis-tutorial4)

the bin centers are defined rp_bin_centers = (rp_bins[:1] + rp_bins[1:])/2. but it should be rp_bin_centers = (rp_bins[:-1] + rp_bins[1:])/2. (with a minus sign)

It's minor but lead to some confusion.

[aphearin]

Thanks for catching this! I'll be sure to fix before the v0.7 release.