minor update to the readme

Signed-off-by: Michal Wasilewski <mwasilewski@gmx.com>

README.md

@@ -51,11 +51,11 @@ ensure the `fixtures` directory is up to date by removing the existing directory
 extracting the ttar file using `make fixtures/.unpacked` or just `make test`.
 
 ```bash
-rm -rf fixtures
+rm -rf testdata/fixtures
 make test
 ```
 
 Next, make the required changes to the extracted files in the `fixtures` directory.  When
 the changes are complete, run `make update_fixtures` to create a new `fixtures.ttar` file
 based on the updated `fixtures` directory.  And finally, verify the changes using
-`git diff fixtures.ttar`.
+`git diff testdata/fixtures.ttar`.