docs: clarify “dag” in std::sys_common doc comment

Author: ColtenOuO

library/std/src/sys_common/mod.rs

@@ -11,7 +11,7 @@
 //! This is because `sys_common` not only contains platform-independent code,
 //! but also code that is shared between the different platforms in `sys`.
 //! Ideally all that shared code should be moved to `sys::common`,
-//! and the dependencies between `std`, `sys_common` and `sys` all would form a dag.
+//! and the dependencies between `std`, `sys_common` and `sys` all would form a DAG.
 //! Progress on this is tracked in #84187.
 
 #![allow(missing_docs)]