DOC: tweak for correctness

closes gh-328

Author: lucascolley

README.md

@@ -42,7 +42,7 @@ To specify a runtime-defined module, define `xp` using the `exec('...')` syntax:
 $ export ARRAY_API_TESTS_MODULE="exec('import quantity_array, numpy; xp = quantity_array.quantity_namespace(numpy)')"
 ```
 
-Alternately, import/define the `xp` variable in `array_api_tests/__init__.py`.
+Alternately, import/define the `xp` and `xp_name` variables in `array_api_tests/__init__.py`.
 
 ### Specifying the API version