dev-handbook/tools: fix Emacs reference URL and some grammar

1. Currently the given link of Emacs tutorial is broken and outdated.

Emacs 19 was released way before and the FTP server is also unavailable.

As these links get out of date rather quickly, use the online reference
URL instead.

2. Fix grammar, use present tense (to match "can") and remove "an".

(e.g. ran -> run, an another -> another)

Reviewed by: Mina Galić <freebsd@igalic.co>, dbaio
Pull Request: https://github.com/freebsd/freebsd-doc/pull/332
Signed-off-by: rilysh <nightquick@proton.me>