Update README message for correctness

README.md

@@ -42,7 +42,7 @@ export LLVM_SYS_170_PREFIX=/usr/lib/llvm-17
 export TABLEGEN_170_PREFIX=/usr/lib/llvm-17
 ```
 
-If you installed llvm with brew, set up the env vars with the following script:
+If you installed llvm with brew, source the `env-macos.sh` script to set up the needed env vars:
 ```sh
 source scripts/env-macos.sh
 ```