docs: Fixed incorrect wording Update firewalls-and-subnetting.md

[0xsimka]

In the "Firewalls and subnetting" section, I corrected the phrase:

"It is heavily recommended to have one configured to improve the security of your node/validator setup."

The word "heavily" was incorrectly used here. The correct version is:

"It is strongly recommended to have one configured to improve the security of your node/validator setup."

This small change improves the clarity and correctness of the sentence.