Update gpg verification instructions for Linux users on our verifying signatures page

Doing a

gpg --verify Downloads/tor-browser-linux64-6.5_en-US.tar.xz{.asc*,}

seems to break under some circumstances for Linux users yet we recommend something like that on our website where we explain how to verify the signatures of freshly downloaded Tor Browser bundles.

It seems to me doing something like

gpg --verify tor-browser-linux64-6.5_en-US.tar.xz.asc tor-browser-linux64-6.5_en-US.tar.xz

is a saner approach.