Fix issue label link in docs/dev/README.md

Author: pcpthm

docs/dev/README.md

@@ -45,7 +45,7 @@ mandatory), add yourself to the list!
   [E-medium](https://github.com/rust-analyzer/rust-analyzer/issues?q=is%3Aopen+is%3Aissue+label%3AE-medium),
   [E-hard](https://github.com/rust-analyzer/rust-analyzer/issues?q=is%3Aopen+is%3Aissue+label%3AE-hard),
   labels are *estimates* for how hard would be to write a fix.
-* [E-fun](https://github.com/rust-analyzer/rust-analyzer/issues?q=is%3Aopen+is%3Aissue+label%3AE-fun)
+* [fun](https://github.com/rust-analyzer/rust-analyzer/issues?q=is%3Aopen+is%3Aissue+label%3Afun)
   is for cool, but probably hard stuff.
 
 # CI