use a better emoji for under construction

Author: nikomatsakis

src/SUMMARY.md

@@ -23,7 +23,7 @@
 
 ## Rust 2021
 
-- [Rust 2021 ⚠️](rust-2021/index.md)
+- [Rust 2021 🚧](rust-2021/index.md)
   - [Additions to the prelude](rust-2021/prelude.md)
   - [Default Cargo feature resolver](rust-2021/default_cargo_resolver.md)
   - [IntoIterator for arrays](rust-2021/IntoIterator_for_arrays.md)

src/rust-2021/index.md

@@ -1,6 +1,6 @@
-# Rust 2021 ⚠️
+# Rust 2021
 
-⚠️ The 2021 Edition has not yet been released and hence this section is still "under construction".
+🚧 The 2021 Edition has not yet been released and hence this section is still "under construction".
 
 The Rust 2021 Edition is currently slated for release in Rust 1.56.0.
 Rust 1.56.0 will then be in beta for six weeks,