readme: fix echo mode note

Author: paulirish

README.md

@@ -144,7 +144,7 @@ Please provide examples of the URLs you are parsing with each PR.
 
 You can run `git-open` in `echo` mode, which doesn't open your browser, but just prints the URL to stdout:
 ```sh
-BATS_CWD="." ./git-open
+env BROWSER='echo' ./git-open
 ```
 
 #### Testing: