field projections: fix url encoding

The `@BennoLossin` in the hackmd url gets interpreted as a github
username and thus replaced by the url, which breaks the markdown.

Thus use `%40` which encodes `@` instead.

Author: BennoLossin

src/2025h2/field-projections.md

@@ -74,7 +74,7 @@ that improves upon it.
 
 For historical context, there also is the [Field Projections RFC v1].
 
-[newer proposal]: https://hackmd.io/@BennoLossin/HkMBy6Hzlx
+[newer proposal]: https://hackmd.io/%40BennoLossin/HkMBy6Hzlx
 [Field Projections RFC v1]: https://github.com/rust-lang/rfcs/pull/3318
 [Field Projections RFC v2]: https://github.com/rust-lang/rfcs/pull/3735