Update release notes

ReleaseNotes.md

@@ -1,7 +1,7 @@
 # Release Notes—NJOY2016
 Given here are some release notes for NJOY2016. Each release is made through a formal [Pull Request](https://github.com/njoy/NJOY2016/pulls) made on GitHub. There are links in this document that point to each of those Pull Requests, where you can see in great details the changes that were made. Often the Pull Requests are made in response to an [issue](https://github.com/njoy/NJOY2016/issues). In such cases, links to those issues are also given.
 
-## [NJOY2016.73](https://github.com/njoy/NJOY2016/pull/xxx)
+## [NJOY2016.73](https://github.com/njoy/NJOY2016/pull/321)
 This update fixes the following issues:
   - Fix an issue in ACER for thermal scattering leading to energy values being out of order when plotting the coherent elastic scattering cross section (this issue only affects plots, the thermal scattering ACE files do not change).
   - Increased allocation of an array in LEAPR to accommodate ENDF/B-VIII.1 thermal scattering evaluations and added a check to avoid an infinite loop when using a very fine beta grid. In addition, LEAPR will now warn the user about potential excessive calculation times and print out progression in the phonon expansion sum when the phonon expansion order is large.