Unbalanced parenthesis in -F/--filter-grades help text

[dbosk]

Context

Batch 4 of the bug-fix campaign, while widening the grade filter regexes (PR #394).

Observation

The -F help string in results.nw (<<add option to include Fs>>) reads f"A--Es and Ps ({PASSING_REGEX}. " — the opening parenthesis is never closed.

Why it matters

Cosmetic, but user-visible in canvaslms results -h.

Suggested follow-up

One-character fix: close the parenthesis after the regex.