Prepare RFC 3593 to be merged

The FCP for RFC 3593 has completed, so let's prepare it to be merged.
In this case, that means changing the file name to use dashes rather
than underscores as separators.

Author: traviscross

text/3593-unprefixed_guarded_strings.md renamed to text/3593-unprefixed-guarded-strings.md

@@ -1,7 +1,7 @@
 - Feature Name: `unprefixed_guarded_strings`
 - Start Date: 2024-03-24
 - RFC PR: [rust-lang/rfcs#3593](https://github.com/rust-lang/rfcs/pull/3593)
-- Rust Issue: [rust-lang/rust#3593](https://github.com/rust-lang/rust/issues/3593)
+- Tracking Issue: [rust-lang/rust#3593](https://github.com/rust-lang/rust/issues/3593)
 
 # Summary
 [summary]: #summary