doc: Fix typo in zulipbot-usage doc.
doc: Fix typo in zulipbot-usage doc.

Fix a spelling mistake in the zulipbot doc.

eeshangarg/zulip,andersk/zulip,zulip/zulip,rht/zulip,kou/zulip,zulip/zulip,eeshangarg/zulip,rht/zulip,zulip/zulip,eeshangarg/zulip,eeshangarg/zulip,andersk/zulip,eeshangarg/zulip,zulip/zulip,kou/zulip,zulip/zulip,zulip/zulip,andersk/zulip,rht/zulip,kou/zulip,kou/zulip,andersk/zulip,andersk/zulip,rht/zulip,kou/zulip,zulip/zulip,rht/zulip,kou/zulip,rht/zulip,rht/zulip,kou/zulip,eeshangarg/zulip,eeshangarg/zulip,andersk/zulip,andersk/zulip