docs: Fix some broken links in security model doc.
docs: Fix some broken links in security model doc.

Apparently, we haven't been running test-documentation in production
of late.

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