add PORT Procfile

[CarBoss1996]

Procfileに環境変数　PORT　を記述

変更前 web: env RUBY_DEBUG_OPEN=true bin/rails server -b 0.0.0.0 -p 3000

変更後 web: env RUBY_DEBUG_OPEN=true bin/rails server -b 0.0.0.0 -p ${PORT:-3000}