Update Index.rst

Solve Sphinx warning about non ending </strong> part.

Documentation/Administrator/Upgrade/Index.rst

@@ -17,5 +17,5 @@ We have removed some properties from Scheduler Task classes. Please test,
 if your tasks are still running. If not, you have to remove the task and
 create it again.
 
-We have migrated the *.ts files files to *.typoscript. Please update
+We have migrated the file ending `ts` to `typoscript`. Please update
 your references, if you make use of the old file endings.