SMTP notify: Use grep -E instead of expr

[chris03]

expr was printing expr: warning: '^.*[<>"]': using '^' as the first character of a basic regular expression is not portable;

To reproduce the issue:

docker run --rm -e SMTP_FROM=alain@example.com alpine:latest sh -c 'apk add curl openssl && curl https://get.acme.sh/ | sh -s email=bob@example.com && /root/.acme.sh/acme.sh --set-notify --notify-hook smtp'

[github-actions]

Welcome
Please make sure you're read our Code-of-conduct and add the usage here: notify.
Then reply on this message, otherwise, your code will not be reviewed or merged.
We look forward to reviewing your Pull request shortly ✨

[Neilpang]

fix the shellcheck errors