Add reference implemention link

Author: clavin

text/0012-corner-smoothing.md

@@ -3,7 +3,7 @@
 - Start Date: 2024-11-26
 - RFC PR: [electron/rfcs#0012](https://github.com/electron/rfcs/pull/12)
 - Electron Issues: None <!-- [electron/electron#0000](https://github.com/electron/electron/issue/0000) -->
-- Reference Implementation: None (yet) <!-- [electron/electron#0000](https://github.com/electron/electron/pull/0000) -->
+- Reference Implementation: [electron/electron#45185](https://github.com/electron/electron/pull/45185)
 - Status: **Proposed**
 
 # Corner Smoothing