webhook-walkthrough: Replace references to old style fixture names.
webhook-walkthrough: Replace references to old style fixture names.

The webhook-walkthrough has now been updated to reflect the recent
decision to rename webhook fixtures from
<webhook_name>/fixtures/<webhook_name>_<event_type>.json to
<webhook_name>/fixtures/<event_type>.json.

rht/zulip,shubhamdhama/zulip,jackrzhang/zulip,verma-varsha/zulip,tommyip/zulip,vaidap/zulip,Galexrt/zulip,zulip/zulip,zulip/zulip,amanharitsh123/zulip,kou/zulip,verma-varsha/zulip,ryanbackman/zulip,zulip/zulip,vaidap/zulip,j831/zulip,showell/zulip,jackrzhang/zulip,dhcrzf/zulip,kou/zulip,andersk/zulip,hackerkid/zulip,amanharitsh123/zulip,rht/zulip,jrowan/zulip,jphilipsen05/zulip,tommyip/zulip,timabbott/zulip,kou/zulip,kou/zulip,verma-varsha/zulip,dhcrzf/zulip,vabs22/zulip,timabbott/zulip,andersk/zulip,amanharitsh123/zulip,vabs22/zulip,eeshangarg/zulip,verma-varsha/zulip,jackrzhang/zulip,showell/zulip,brockwhittaker/zulip,shubhamdhama/zulip,hackerkid/zulip,j831/zulip,vabs22/zulip,jackrzhang/zulip,mahim97/zulip,zulip/zulip,punchagan/zulip,vabs22/zulip,showell/zulip,brainwane/zulip,shubhamdhama/zulip,eeshangarg/zulip,ryanbackman/zulip,andersk/zulip,dhcrzf/zulip,jackrzhang/zulip,punchagan/zulip,rishig/zulip,hackerkid/zulip,rht/zulip,christi3k/zulip,synicalsyntax/zulip,showell/zulip,rishig/zulip,timabbott/zulip,brainwane/zulip,amanharitsh123/zulip,christi3k/zulip,rht/zulip,timabbott/zulip,timabbott/zulip,rht/zulip,tommyip/zulip,eeshangarg/zulip,brainwane/zulip,jphilipsen05/zulip,brockwhittaker/zulip,tommyip/zulip,timabbott/zulip,mahim97/zulip,tommyip/zulip,rishig/zulip,jphilipsen05/zulip,eeshangarg/zulip,jrowan/zulip,vabs22/zulip,shubhamdhama/zulip,jrowan/zulip,showell/zulip,brainwane/zulip,ryanbackman/zulip,punchagan/zulip,Galexrt/zulip,christi3k/zulip,Galexrt/zulip,brockwhittaker/zulip,showell/zulip,kou/zulip,shubhamdhama/zulip,brainwane/zulip,rishig/zulip,synicalsyntax/zulip,hackerkid/zulip,rishig/zulip,zulip/zulip,ryanbackman/zulip,j831/zulip,j831/zulip,vaidap/zulip,jphilipsen05/zulip,j831/zulip,brockwhittaker/zulip,synicalsyntax/zulip,jrowan/zulip,brockwhittaker/zulip,timabbott/zulip,vabs22/zulip,jrowan/zulip,zulip/zulip,vaidap/zulip,brainwane/zulip,brainwane/zulip,amanharitsh123/zulip,ryanbackman/zulip,punchagan/zulip,dhcrzf/zulip,andersk/zulip,synicalsyntax/zulip,j831/zulip,amanharitsh123/zulip,shubhamdhama/zulip,verma-varsha/zulip,Galexrt/zulip,vaidap/zulip,andersk/zulip,jrowan/zulip,synicalsyntax/zulip,andersk/zulip,showell/zulip,dhcrzf/zulip,Galexrt/zulip,mahim97/zulip,tommyip/zulip,hackerkid/zulip,dhcrzf/zulip,zulip/zulip,eeshangarg/zulip,mahim97/zulip,jackrzhang/zulip,verma-varsha/zulip,rishig/zulip,synicalsyntax/zulip,andersk/zulip,kou/zulip,rishig/zulip,mahim97/zulip,rht/zulip,christi3k/zulip,jphilipsen05/zulip,jphilipsen05/zulip,tommyip/zulip,christi3k/zulip,brockwhittaker/zulip,punchagan/zulip,rht/zulip,punchagan/zulip,shubhamdhama/zulip,eeshangarg/zulip,Galexrt/zulip,jackrzhang/zulip,ryanbackman/zulip,hackerkid/zulip,Galexrt/zulip,kou/zulip,vaidap/zulip,synicalsyntax/zulip,christi3k/zulip,dhcrzf/zulip,hackerkid/zulip,mahim97/zulip,eeshangarg/zulip,punchagan/zulip