Rename 'expiration.on-sync' to 'purge.on-sync' (#3556)

Taskwarrior uses "expire" to refer to deletion of tasks past their
"until" date, so let's use `purge` to link this semantically to the
`task purge` command.

src/commands/CmdShow.cpp

@@ -160,7 +160,7 @@ int CmdShow::execute (std::string& output)
     " due"
     " editor"
     " exit.on.missing.db"
-    " expiration.on-sync"
+    " purge.on-sync"
     " expressions"
     " fontunderline"
     " gc"