RFC 2963 is rustdoc's JSON backend

Author: Manishearth

text/0000-rustdoc-json.md renamed to text/2963-rustdoc-json.md

@@ -1,7 +1,7 @@
 - Feature Name: `rustdoc_json`
 - Start Date: 2020-06-26
 - RFC PR: [rust-lang/rfcs#2963](https://github.com/rust-lang/rfcs/pull/2963)
-- Rust Issue: [rust-lang/rust#0000](https://github.com/rust-lang/rust/issues/0000)
+- Rust Issue: [rust-lang/rust#0000](https://github.com/rust-lang/rust/issues/76578)
 
 # Summary
 [summary]: #summary