fix: correct image directory path in build script

Author: Chivier

build_html.sh

@@ -6,6 +6,6 @@ rm -rf _build/rst _build/html
 d2lbook build rst
 cp static/frontpage.html _build/rst/
 d2lbook build html
-cp -r static/image _build/html/_image
+cp -r static/image _build/html/_static
 cp -r static/* _build/html/_static
-python3 tools/format_tables.py
+python3 tools/format_tables.py