Wiki: typos and additions in build instructions

[elboulangero]

Hey, I don't think GitHub supports PR for the wiki, so I have to go through the Issues.

https://github.com/elboulangero/gradio-wiki/commit/dcdaedacf117095d019b4aa4e5407c1f4668dade

Cherry-pick that if you feel like !

BTW I didn't manage to build GRadio, I'm stuck at:

# ninja
ninja: error: '../external/gd/libgd/external/gd/libgd/gd-tagged-entry-default.css', needed by 'external/gd/libgd/gd-tagged-entry-resources.c', missing and no known rule to make it

Any idea ? It could be my setup though, I'm running Debian Stretch with a backported version of meson, that could be the issue.

Cheers !

Arnaud