Add a missing PR reference.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1929182 13f79535-47bb-0310-9956-ffa450edef68

Author: notroj

CHANGES

@@ -550,7 +550,8 @@ Changes with Apache 2.4.59
   *) htpasswd: Add support for passwords using SHA-2.  [Joe Orton,
      Yann Ylavic]
 
-  *) core: Allow mod_env to override system environment vars. [Joe Orton]
+  *) core: Allow mod_env to override system environment vars. PR 63117.
+     [Joe Orton]
 
   *) Allow mod_dav_fs to tolerate race conditions between PROPFIND and an
      operation which removes a directory/file between apr_dir_read() and