Fix trailing whitespace in build output

Author: marijnh

rollup.config.js

@@ -11,7 +11,7 @@ export default {
 //
 // You can find some technical background for some of the code below
 // at http://marijnhaverbeke.nl/blog/#cm-internals .
-  `,
+`,
     format: "umd",
     file: "lib/codemirror.js",
     name: "CodeMirror"