tests/test-gendoc-el.t

copies-rust: remove an unnecessary format!() inside assert!()

The `assert!()` macro supports formatting. Rust 2021 no longer
supports an unnecessary `format!()` inside it. I noticed because I
recently update my Rust toolchain and `test-check-cargo-lock.t`
started failing.

Differential Revision: https://phab.mercurial-scm.org/D10085

#require docutils gettext

  $ $TESTDIR/check-gendoc el
  checking for parse errors