Updated tutorial

According to the [change log](https://github.com/vimperator/vimperator-labs/blob/32d5a35a3b75319a365f97b94a8b38887f11f6ea/vimperator/NEWS) for version 3.1:

> IMPORTANT: :viusage, :exusage and :optionusage (and the manually created
index of all commands in the helpfile) have been replaced by a single
:usage command.

So I just updated the outdated xml file.