fix for outdated file reference for excluding countries

[Nageshk88]

What this fixes

Fixes #1377

The Development section of the README instructed users to remove
unwanted countries from sass/_flag-icons-list.scss. That file no
longer stores the country list directly — it now just loops over the
$flag-icons-included-countries variable, which lives in
sass/_variables.scss.

This means the instruction was inconsistent with the Usage section
above it, which already correctly points to sass/_variables.scss.

Change

Updated the Development section to reference sass/_variables.scss
and the $flag-icons-included-countries variable instead of the
outdated _flag-icons-list.scss file, so both sections of the README
now agree.

Type of change

• Documentation fix (no functional/code changes)

Checklist

• I've read the relevant section of the README before and after the change
• This is a docs-only change; no build step required
• Linked the related issue (Error in README #1377)