Add VAPID keys section to needed variables section (#385)

VAPID keys are necessary for Web Push notifications to work.

Also fix a small typo.

Running-Mastodon/Production-guide.md

@@ -399,11 +399,16 @@ LOCAL_DOMAIN=example.com
 LOCAL_HTTPS=true
 
 # Application secrets
-# Generate each eith `RAILS_ENV=production bundle exec rake secret`
+# Generate each with `RAILS_ENV=production bundle exec rake secret`
 PAPERCLIP_SECRET=
 SECRET_KEY_BASE=
 OTP_SECRET=
 
+# Web Push VAPID keys
+# Generate with `RAILS_ENV=production bundle exec rake mastodon:webpush:generate_vapid_key`
+VAPID_PRIVATE_KEY=
+VAPID_PUBLIC_KEY=
+
 # All SMTP details, Mailgun and Sparkpost have free tiers
 SMTP_SERVER=
 SMTP_PORT=