Use cargo rustc instead of setting RUSTFLAGS

Author: theotherphil

src/hir.md

@@ -11,11 +11,10 @@ been desugared away (as an example, `for` loops are converted into a
 This chapter covers the main concepts of the HIR.
 
 You can view the HIR representation of your code by passing the
-`-Zunpretty=hir-tree` flag to rustc, for example by setting the `RUSTFLAGS`
-environment variable:
+`-Zunpretty=hir-tree` flag to rustc:
 
 ```
-RUSTFLAGS=-Zunpretty=hir-tree cargo build
+cargo rustc -- -Zunpretty=hir-tree
 ```
 
 ### Out-of-band storage and the `Crate` type