Fix reference to no-longer-existent 'rollup.config.js' file

ExplodingCabbage · ExplodingCabbage · commit 9b003888072b · 2024-08-20T17:51:01.000+01:00
Should've been done as part of #505, but I missed it
diff --git a/.npmignore b/.npmignore
@@ -10,7 +10,7 @@ Gruntfile.js
 images
 index.html
 karma.conf.js
-rollup.config.js
+rollup.config.mjs
 src
 tasks
 test
