Use https for lunrjs.com link in header

I use https://lychee.cli.rs/ to check for broken links, and it recently
started linting for http links that could be https in generated docs.

I could also just ignore the error, but hopefully fixing it here means
other folks don't have to do so too.

```
[build/assets/js/540.9d944f2a.js.LICENSE.txt]:
   [ERROR] http://lunrjs.com/ | This URI is available in HTTPS protocol, but HTTP is provided. Use 'https://lunrjs.com/' instead
```

Author: gibfahn

build/wrapper_start

@@ -1,5 +1,5 @@
 /**
- * lunr - http://lunrjs.com - A bit like Solr, but much smaller and not as bright - @VERSION
+ * lunr - https://lunrjs.com - A bit like Solr, but much smaller and not as bright - @VERSION
  * Copyright (C) @YEAR Oliver Nightingale
  * @license MIT
  */

lunr.js

@@ -1,5 +1,5 @@
 /**
- * lunr - http://lunrjs.com - A bit like Solr, but much smaller and not as bright - 2.3.9
+ * lunr - https://lunrjs.com - A bit like Solr, but much smaller and not as bright - 2.3.9
  * Copyright (C) 2020 Oliver Nightingale
  * @license MIT
  */