register_server: Fix recommendation to run with python 2.
register_server: Fix recommendation to run with python 2.

Since `python` is usually Python 2, this recommendation failed on most
systems.

eeshangarg/zulip,timabbott/zulip,showell/zulip,kou/zulip,hackerkid/zulip,rishig/zulip,brainwane/zulip,dhcrzf/zulip,eeshangarg/zulip,showell/zulip,kou/zulip,brainwane/zulip,zulip/zulip,jackrzhang/zulip,synicalsyntax/zulip,kou/zulip,timabbott/zulip,zulip/zulip,jackrzhang/zulip,shubhamdhama/zulip,rishig/zulip,timabbott/zulip,shubhamdhama/zulip,andersk/zulip,rishig/zulip,dhcrzf/zulip,brainwane/zulip,kou/zulip,brainwane/zulip,showell/zulip,timabbott/zulip,hackerkid/zulip,rht/zulip,kou/zulip,synicalsyntax/zulip,hackerkid/zulip,andersk/zulip,eeshangarg/zulip,eeshangarg/zulip,rht/zulip,hackerkid/zulip,jackrzhang/zulip,shubhamdhama/zulip,brainwane/zulip,rht/zulip,rishig/zulip,rht/zulip,zulip/zulip,andersk/zulip,zulip/zulip,andersk/zulip,rht/zulip,punchagan/zulip,punchagan/zulip,showell/zulip,jackrzhang/zulip,punchagan/zulip,synicalsyntax/zulip,shubhamdhama/zulip,dhcrzf/zulip,kou/zulip,dhcrzf/zulip,shubhamdhama/zulip,brainwane/zulip,jackrzhang/zulip,zulip/zulip,shubhamdhama/zulip,andersk/zulip,shubhamdhama/zulip,showell/zulip,tommyip/zulip,rishig/zulip,hackerkid/zulip,tommyip/zulip,timabbott/zulip,jackrzhang/zulip,tommyip/zulip,rht/zulip,eeshangarg/zulip,showell/zulip,punchagan/zulip,synicalsyntax/zulip,synicalsyntax/zulip,kou/zulip,synicalsyntax/zulip,dhcrzf/zulip,andersk/zulip,hackerkid/zulip,dhcrzf/zulip,brainwane/zulip,rht/zulip,hackerkid/zulip,jackrzhang/zulip,rishig/zulip,punchagan/zulip,rishig/zulip,punchagan/zulip,andersk/zulip,showell/zulip,tommyip/zulip,zulip/zulip,dhcrzf/zulip,tommyip/zulip,timabbott/zulip,synicalsyntax/zulip,tommyip/zulip,timabbott/zulip,punchagan/zulip,zulip/zulip,eeshangarg/zulip,eeshangarg/zulip,tommyip/zulip