Update petition_petition.xml

Added to first paragraph: "For an in depth anaysis of these documents see [Baetens 2020]." Link to biblio ref = https://www.trismegistos.org/top/index.php?#topss

data/descriptions/petition_petition.xml

@@ -77,7 +77,8 @@
                         grammateus</term>, prefect, <term xml:lang="lat">strategos</term>, <term
                         xml:lang="lat">epistrategos</term>, and local officials, as well as military
                     officials such as a centurion, decurion, or <term xml:lang="lat"
-                        >beneficarius</term>. This description will concentrate on non-royal
+                        >beneficarius</term>. For an in depth anaysis of these documents see [Baetens 2020]. 
+                        This description will concentrate on non-royal
                     petitions; royal petitions will be treated separately [<ref type="descr"
                         target="#gt_enteuxis">enteuxis</ref>]. </p>