use proper overflow (#6926)

frontend/src/metabase/visualizations/visualizations/Text.css

@@ -26,7 +26,7 @@
   box-shadow: 0 1px 7px rgba(0, 0, 0, .1);
 }
 :local .Text .text-card-markdown {
-  overflow: scroll;
+  overflow: auto;
   pointer-events: all
 }