docs: Fix setup-advanced git clone instructions.
docs: Fix setup-advanced git clone instructions.

This should help ensure everyone uses the SSH key approach for Git
authentication; the HTTPS one is basically unusable as one has to
provide one's GitHub password after every command.

showell/zulip,synicalsyntax/zulip,eeshangarg/zulip,hackerkid/zulip,synicalsyntax/zulip,showell/zulip,eeshangarg/zulip,showell/zulip,hackerkid/zulip,brainwane/zulip,punchagan/zulip,rht/zulip,punchagan/zulip,shubhamdhama/zulip,rht/zulip,timabbott/zulip,zulip/zulip,timabbott/zulip,showell/zulip,andersk/zulip,zulip/zulip,zulip/zulip,brainwane/zulip,rht/zulip,punchagan/zulip,andersk/zulip,zulip/zulip,synicalsyntax/zulip,shubhamdhama/zulip,zulip/zulip,brainwane/zulip,showell/zulip,hackerkid/zulip,eeshangarg/zulip,eeshangarg/zulip,hackerkid/zulip,kou/zulip,kou/zulip,timabbott/zulip,timabbott/zulip,hackerkid/zulip,brainwane/zulip,rht/zulip,punchagan/zulip,andersk/zulip,hackerkid/zulip,punchagan/zulip,kou/zulip,andersk/zulip,shubhamdhama/zulip,kou/zulip,timabbott/zulip,eeshangarg/zulip,synicalsyntax/zulip,synicalsyntax/zulip,zulip/zulip,andersk/zulip,shubhamdhama/zulip,punchagan/zulip,shubhamdhama/zulip,shubhamdhama/zulip,showell/zulip,punchagan/zulip,rht/zulip,andersk/zulip,eeshangarg/zulip,kou/zulip,brainwane/zulip,synicalsyntax/zulip,hackerkid/zulip,kou/zulip,rht/zulip,kou/zulip,eeshangarg/zulip,zulip/zulip,brainwane/zulip,timabbott/zulip,shubhamdhama/zulip,rht/zulip,timabbott/zulip,synicalsyntax/zulip,showell/zulip,brainwane/zulip,andersk/zulip