Fix typo

Author: moomoohk

tox.ini

@@ -13,7 +13,7 @@ envlist = py36, pypy37, py38, py39, flake8
 python =
     3.6: py36
     # TODO(moomoohk): Get pypy to work properly. https://github.com/ymyzk/run-tox-gh-actions/issues/3
-    pypi-3.7: pypy37
+    pypy-3.7: pypy37
     3.8: py38
     3.9: py39, flake8