fix: Add RFC number

Author: epage

text/0000-libtest-json.md renamed to text/3558-libtest-json.md

@@ -1,7 +1,7 @@
 - Feature Name: `libtest-json`
 - Start Date: 2024-01-18
 - Pre-RFC: [Internals](https://internals.rust-lang.org/t/path-for-stabilizing-libtests-json-output/20163)
-- eRFC PR: [rust-lang/rfcs#0000](https://github.com/rust-lang/rfcs/pull/0000)
+- eRFC PR: [rust-lang/rfcs#3558](https://github.com/rust-lang/rfcs/pull/3558)
 - Rust Issue: [rust-lang/rust#0000](https://github.com/rust-lang/rust/issues/0000)
 
 # Summary