test-api: Fix typo in message event types.
test-api: Fix typo in message event types.

The event type for messages is "message", not "messages" as the test
(and hence the API docs Python example) reflected.

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