Add missing trailing : to ✨ in the PR template

Such a tiny detail, I'm wondering if it's worth spending people time on it.
Sorry!

Author: dlespiau

.github/pull_request_template.md

@@ -1,4 +1,4 @@
 <!-- please add a icon to the title of this PR (see VERSIONING.md), and delete this line and similar ones -->
-<!-- the icon will be either ⚠ (:warning:, major), ✨ (:sparkles, minor), 🐛 (:bug:, patch), 📖 (:book:, docs), or 🏃 (:running:, other) -->
+<!-- the icon will be either ⚠ (:warning:, major), ✨ (:sparkles:, minor), 🐛 (:bug:, patch), 📖 (:book:, docs), or 🏃 (:running:, other) -->
 
 <!-- What does this do, and why do we need it? -->