Set RFC PR number for 3931

Author: ehuss

text/0000-rfmf-rust-foundation-maintainer-fund.md

@@ -1,6 +1,6 @@
 - Feature Name: `rust_foundation_maintainer_fund`
 - Start Date: 2026-02-23
-- RFC PR: [rust-lang/rfcs#0000](https://github.com/rust-lang/rfcs/pull/0000)
+- RFC PR: [rust-lang/rfcs#3931](https://github.com/rust-lang/rfcs/pull/3931)
 
 ## Summary
 [summary]: #summary