fix AdtDef docs

Author: lcnr

src/librustc_middle/ty/mod.rs

@@ -1846,7 +1846,7 @@ pub struct FieldDef {
 
 /// The definition of a user-defined type, e.g., a `struct`, `enum`, or `union`.
 ///
-/// These are all interned (by `intern_adt_def`) into the `adt_defs` table.
+/// These are all interned (by `alloc_adt_def`) into the global arena.
 ///
 /// The initialism *ADT* stands for an [*algebraic data type (ADT)*][adt].
 /// This is slightly wrong because `union`s are not ADTs.