Fix demoweb installation with new composer location

This was also fixed in the crontab on domjudge.org for various commands.
The composer file is now in the webapp folder in the main repo.

update_demoweb.sh

@@ -19,7 +19,7 @@ make QUIET=1 maintainer-clean
 make QUIET=1 MAINT_CXFLAGS='-g -O2 -Wall -fPIE -Wformat -Wformat-security' \
 	inplace-conf >/dev/null 2>&1 || true
 make QUIET=1 inplace-install docs >/dev/null 2>&1
-composer auto-scripts >/dev/null 2>&1
+(cd webapp && composer auto-scripts >/dev/null 2>&1)
 
 # Reset database to known good state:
 mysql $MYSQLOPTS "$DBNAME" < domjudge_demo_remove.sql