doc: fix double link

Author: rugk

CONTRIBUTING.md

@@ -17,7 +17,7 @@ You like this add-on, but have no idea how to support us?
 Here are some easy things you can always do:
 
 * Spread the word and recommend it to others! 🤗😍
-* Leave a rating [at addons.mozilla.org](https://addons.mozilla.org/firefox/addon/offline-qr-code-generator/reviews/?utm_source=github.com&utm_medium=git&&utm_content=contributor-doc-rate-extension&utm_campaign=github-contributor-doc-rate-extension) if you like it!
+* Leave a rating [at addons.mozilla.org](https://addons.mozilla.org/firefox/addon/offline-qr-code-generator/reviews/?utm_source=github.com&utm_medium=git&utm_content=contributor-doc-rate-extension&utm_campaign=github-contributor-doc-rate-extension) if you like it!
   Also consider writing some text and not only leaving stars there. It's always nice to hear some warm words. ☺️
 * Star this project [on GitHub](https://github.com/rugk/offline-qr-code) by clicking the "star" icon!
 * Leave a [review at alternativeto.net](https://alternativeto.net/software/offline-qr-code-generator/reviews/) and click the like button there, if you want. (Also, you can obviously recommend this add-on as an alternative to other emoji pickers.)