Use previous blog post index

src/freedombone-mesh-blog

         return
     fi
     CURRENT_INDEX=$(cat $CURRENT_BLOG_INDEX)
-    LAST_BLOG_ENTRY=$BLOG_CONTENT_PATH/${CURRENT_INDEX}_post.rst
+    PREVIOUS_INDEX=$((CURRENT_INDEX - 1))
+    LAST_BLOG_ENTRY=$BLOG_CONTENT_PATH/${PREVIOUS_INDEX}_post.rst
     if [ ! -f $LAST_BLOG_ENTRY ]; then
         return
     fi