Use unpretty for debugging Hir (#1685)

Author: chenyukang

src/hir-debugging.md

@@ -3,6 +3,6 @@
 The `-Z unpretty=hir-tree` flag will dump out the HIR.
 
 If you are trying to correlate `NodeId`s or `DefId`s with source code, the
-`--pretty expanded,identified` flag may be useful.
+`-Z unpretty=expanded,identified` flag may be useful.
 
 TODO: anything else? [#1159](https://github.com/rust-lang/rustc-dev-guide/issues/1159)