management: Fix incorrect help text in compare_messages.
management: Fix incorrect help text in compare_messages.

This was added, along with `render_messages`, in
93965a8e892227ae7eeab960177320a43c10e1a6.

kou/zulip,eeshangarg/zulip,zulip/zulip,hackerkid/zulip,hackerkid/zulip,punchagan/zulip,andersk/zulip,kou/zulip,punchagan/zulip,hackerkid/zulip,zulip/zulip,andersk/zulip,zulip/zulip,punchagan/zulip,rht/zulip,punchagan/zulip,andersk/zulip,eeshangarg/zulip,rht/zulip,andersk/zulip,zulip/zulip,zulip/zulip,zulip/zulip,rht/zulip,kou/zulip,hackerkid/zulip,eeshangarg/zulip,kou/zulip,eeshangarg/zulip,punchagan/zulip,eeshangarg/zulip,hackerkid/zulip,punchagan/zulip,rht/zulip,rht/zulip,kou/zulip,andersk/zulip,rht/zulip,hackerkid/zulip,rht/zulip,eeshangarg/zulip,punchagan/zulip,eeshangarg/zulip,andersk/zulip,hackerkid/zulip,kou/zulip,zulip/zulip,andersk/zulip,kou/zulip