[doc] fix RDF equation (and hole link) (#4591)

* [docs] fix normalization in g_ab() RDF equation

- doc fix
- see #4437 (reply in thread) (originally found by @rodpollet)

* [doc] fix missing HOLE link

package/MDAnalysis/analysis/rdf.py

@@ -32,7 +32,7 @@
 
 .. math::
 
-   g_{ab}(r) = (N_{a} N_{b})^{-1} \sum_{i=1}^{N_a} \sum_{j=1}^{N_b}
+   g_{ab}(r) = \frac{1}{N_{a}} \frac{1}{N_{b}/V} \sum_{i=1}^{N_a} \sum_{j=1}^{N_b}
                \langle \delta(|\mathbf{r}_i - \mathbf{r}_j| - r) \rangle
 
 which is normalized so that the RDF becomes 1 for large separations in a

package/doc/sphinx/source/documentation_pages/analysis/hole2.rst

@@ -24,6 +24,8 @@ under the Apache v2.0 license.)
    `mdahole2 <https://www.mdanalysis.org/mdahole2/>`_ and
    will be removed in MDAnalysis 3.0.0.
 
+.. _HOLE: http://www.holeprogram.org/
+
 
 See Also
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