Standardize on `exit N` vs `exit(N)` style in example solutions

[kirtisrivastava22]

Follow-up from #431 — glennj noted that example solutions inconsistently mix exit 1 and exit(1) styles (the parens aren't idiomatic AWK, similar to the print(...) issue fixed in #431/#440).

Happy to submit a PR standardizing on whichever style maintainers prefer.