"Getting Zephyr" page is out of date.

[smanilov]

The page available at https://risc-v-getting-started-guide.readthedocs.io/en/latest/getting-zephyr.html seems to be out of date. It refers to version 10.0, while current zephyr is 16.5. I tried editing the page, but it seems too convoluted for a simple fix.