Update Installation Instructions for Linux

As per request by @gus, below is a user's concern and resolution regarding the instructions for Linux:

Hello,

I believe the instructions under the "For GNU/Linux:" section of "https:/tb-manual.tor-project.org/installation/" could be incorrect.

On Ubuntu 20.04 it worked until step 5 where I was required to right click on "start-tor-browser.desktop" and make it executable.

It turns out (for me at least) that this does not work, instead of launching tor-browser it simply launches the file in a text editor.

After extracting with "tar -xf" a folder was created called tor-browser_en-US. Inside it was the .desktop file and a folder named "Browser". Inside the Browser folder was a file named start-tor-browser (it had no extension). I had to make that file executable and then move the .desktop file to ~/.local/share/applications. From there i could find it by amongst all other applications (more specifically i was able to press super then type tor and the icon would pop-up.