fixup! Add 'caption' to file message

+ with

threema/gateway/bin/gateway_client.py

@@ -428,7 +428,7 @@ async def send_video(ctx, **arguments):
 The relative or absolute path to a thumbnail.
 """)
 @click.option('-c', '--caption', help="""
-An optional caption to send along the attached file.
+An optional caption to send alongside the attached file.
 """)
 @click.pass_context
 @util.aio_run