bugdown: Update outdated comment about Zulip's heading support.
bugdown: Update outdated comment about Zulip's heading support.

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