Use `:network` modifier. Closes #2

The [`:network`](https://man.openbsd.org/pf.conf.5#:network) modifier will pick up the IP address range from `hostname.if`. Using this notation will prevent repetition of hard-coded IP address ranges.

Fix URL to GitHub repo

It's not needed as the unicast reverse path forwarding (URPF) check does the same.

This statement is useless as the connection has already been dealt with in the above match, block and pass statements for ICMP.

I have decided to remove links to translations of this guide that is located outside the source of this project. I don't want to monitor these links. Translations are welcome, but they need to contribute to the source i.e. fr/index.html, ja/index.html

Reviewed-on: unixsheikh/openbsd-router-guide#5

others typo ;-)

Reviewed-on: unixsheikh/openbsd-router-guide#4

some typo(s) :p

Reviewed-on: unixsheikh/openbsd-router-guide#3