Fix sed command

Author: janhohenheim

.github/create_newsletter_scaffold.sh

@@ -27,7 +27,7 @@ cp .github/newsletter-template.md index.md
 sed -i "s/{{\s*NEWSLETTER_MONTH\s*}}/${NEWSLETTER_MONTH}/g" index.md
 sed -i "s/{{\s*NEWSLETTER_YEAR\s*}}/${NEWSLETTER_YEAR}/g" index.md
 sed -i "s/{{\s*NEWSLETTER_COUNTER\s*}}/${NEWSLETTER_COUNTER}/g" index.md
-sed -i "s/{{\s*NEWSLETTER_COUNTER_NEXT\s*}}/$(( ${NEWSLETTER_COUNTER} + 1)/g" index.md
+sed -i "s/{{\s*NEWSLETTER_COUNTER_NEXT\s*}}/$((NEWSLETTER_COUNTER + 1))/g" index.md
 sed -i "s/{{\s*NEWSLETTER_NEXT_MONTH_NUMBER\s*}}/${NEWSLETTER_NEXT_MONTH_NUMBER}/g" index.md
 # Create a dir in content/news with the counter with 3 digits as name
 destination="content/news/$(printf "%03d" ${NEWSLETTER_COUNTER})"