rustdoc: Rename "Type Definition" to "Type Alias"

This matches the name used by the Rust Reference [1], which is also what
people usually call these items.

[1]: https://doc.rust-lang.org/reference/items/type-aliases.html

src/librustdoc/html/render/mod.rs

@@ -2123,7 +2123,7 @@ impl ItemSection {
             Self::Unions => "Unions",
             Self::Enums => "Enums",
             Self::Functions => "Functions",
-            Self::TypeDefinitions => "Type Definitions",
+            Self::TypeDefinitions => "Type Aliases",
             Self::Statics => "Statics",
             Self::Constants => "Constants",
             Self::Traits => "Traits",