docs: remove repeated 'etc.' word from sending-messages.md.
docs: remove repeated 'etc.' word from sending-messages.md.

There are two 'etc' words in the same line. This removes one of them.

Note from Tim: The old text was arguably correct, but removing it is definitely clearer.
tommyip/zulip,zulip/zulip,showell/zulip,rishig/zulip,brainwane/zulip,showell/zulip,eeshangarg/zulip,rht/zulip,hackerkid/zulip,eeshangarg/zulip,andersk/zulip,synicalsyntax/zulip,hackerkid/zulip,zulip/zulip,timabbott/zulip,andersk/zulip,shubhamdhama/zulip,shubhamdhama/zulip,synicalsyntax/zulip,punchagan/zulip,kou/zulip,kou/zulip,rht/zulip,hackerkid/zulip,eeshangarg/zulip,synicalsyntax/zulip,kou/zulip,timabbott/zulip,synicalsyntax/zulip,timabbott/zulip,shubhamdhama/zulip,eeshangarg/zulip,andersk/zulip,brainwane/zulip,punchagan/zulip,timabbott/zulip,tommyip/zulip,andersk/zulip,hackerkid/zulip,brainwane/zulip,synicalsyntax/zulip,rishig/zulip,punchagan/zulip,timabbott/zulip,kou/zulip,zulip/zulip,showell/zulip,hackerkid/zulip,tommyip/zulip,rishig/zulip,zulip/zulip,brainwane/zulip,rishig/zulip,punchagan/zulip,tommyip/zulip,andersk/zulip,rht/zulip,zulip/zulip,kou/zulip,tommyip/zulip,shubhamdhama/zulip,eeshangarg/zulip,rishig/zulip,rishig/zulip,synicalsyntax/zulip,hackerkid/zulip,showell/zulip,rht/zulip,showell/zulip,zulip/zulip,rht/zulip,shubhamdhama/zulip,andersk/zulip,timabbott/zulip,rishig/zulip,brainwane/zulip,showell/zulip,tommyip/zulip,eeshangarg/zulip,shubhamdhama/zulip,eeshangarg/zulip,punchagan/zulip,kou/zulip,hackerkid/zulip,andersk/zulip,zulip/zulip,brainwane/zulip,brainwane/zulip,showell/zulip,tommyip/zulip,synicalsyntax/zulip,punchagan/zulip,shubhamdhama/zulip,timabbott/zulip,kou/zulip,rht/zulip,punchagan/zulip,rht/zulip