chore: small fix for outdated docs (#8821)

Co-authored-by: Tom French <[email protected]>

Author: asterite

compiler/noirc_evaluator/src/ssa/ir/map.rs

@@ -270,7 +270,7 @@ impl<T> std::ops::IndexMut<Id<T>> for SparseMap<T> {
 /// Useful for assigning ids before the storage is created or assigning ids
 /// for types that have no single owner.
 ///
-/// This type wraps an AtomicUsize so it can safely be used across threads.
+/// This type wraps an atomic number so it can safely be used across threads.
 #[derive(Debug, Serialize, Deserialize)]
 pub struct AtomicCounter<T> {
     next: AtomicU32,