Capitalize the "unsafe" in "unsafe extern"

For better parity with "Unsafe attributes", e.g. in the table of
contents, let's similarly capitalize the "unsafe" in "unsafe extern".

src/SUMMARY.md

@@ -50,5 +50,5 @@
   - [`gen` keyword](rust-2024/gen-keyword.md)
   - [Macro fragment specifiers](rust-2024/macro-fragment-specifiers.md)
   - [Never type fallback change](rust-2024/never-type-fallback.md)
-  - [`unsafe extern` blocks](rust-2024/unsafe-extern.md)
+  - [Unsafe `extern` blocks](rust-2024/unsafe-extern.md)
   - [Unsafe attributes](rust-2024/unsafe-attributes.md)

src/rust-2024/unsafe-extern.md

@@ -1,4 +1,4 @@
-# `unsafe extern` blocks
+# Unsafe `extern` blocks
 
 🚧 The 2024 Edition has not yet been released and hence this section is still "under construction".
 More information may be found in the tracking issue at <https://github.com/rust-lang/rust/issues/123743>.