Prepare RFC 3681 to be merged

Author: traviscross

text/0000-default-field-values.md renamed to text/3681-default-field-values.md

@@ -1,7 +1,7 @@
 - Feature Name: `default_field_values`
 - Start Date: 2024-08-22
 - RFC PR: https://github.com/rust-lang/rfcs/pull/3681
-- Rust Issue:
+- Tracking Issue: https://github.com/rust-lang/rust/issues/132162
 
 # Summary
 [summary]: #summary