message: Lowercase docs error message for consistency
diff: @@ -614,7 +614,7 @@ fn rustdoc(cx: &mut Context<'_, '_>, unit: &Unit) -> CargoResult<Work> {
  &mut |line| on_stderr_line(state, line, package_id, &target, &mut output_options),
  false,
  )
  - .chain_err(|| format!("Could not document `{}`.", name))?;
  + .chain_err(|| format!("could not document `{}`.", name))?;
  Ok(())
  }))
  }
  