Update american-journal-of-agricultural-economics.csl

Chenged style so citations with more than for authors use et al. Eg.  Michler, Tjernström, Verkaart, and Mausch (2019) should be cited as Michler et al. (2019). As per May 2020 AJAE checklist.

american-journal-of-agricultural-economics.csl

@@ -110,7 +110,7 @@
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-  <citation et-al-min="4" et-al-use-first="1" et-al-subsequent-min="3" et-al-subsequent-use-first="1" disambiguate-add-year-suffix="true" disambiguate-add-names="true" disambiguate-add-givenname="true">
+  <citation et-al-min="5" et-al-use-first="1" et-al-subsequent-min="5" et-al-subsequent-use-first="1" disambiguate-add-year-suffix="true" disambiguate-add-names="true" disambiguate-add-givenname="true">
     <layout prefix="(" suffix=")" delimiter="; ">
       <group delimiter=", ">
         <group delimiter=" ">