centos-rhel bridge page slightly buggy

On https://community.torproject.org/relay/setup/bridge/centos-rhel/ in Section 5, I'm told we need two fixes:

(1) export GOPATH='mktemp -d' wants backticks rather than normal ticks, because it needs to actually run mktemp.

(2) go get gitlab.com/yawning/obfs4.git/obfs4proxy should be go install gitlab.com/yawning/obfs4.git/obfs4proxy@latest (at least on el 8.7)

Ticket transcribed for the reporter 'akik' on #tor-relays channel.

(Addendum from me: For 2, we might want to point people to Tor's obfs4proxy git, not yawning's, too?)