[basics/ribbon] switch to main instead of master

basics.html

@@ -1156,7 +1156,7 @@
 
     <div class="ribbon-wrapper right">
       <div class="ribbon">
-        <a href="https://git.idiv.de/sc/edu/git-seminar/edit/master/basics.html"
+        <a href="https://git.idiv.de/sc/edu/git-seminar/edit/main/basics.html"
             target="_blank">
           edit
         </a>