Fix typo

CONTRIBUTING.rst

@@ -111,7 +111,7 @@ Testing tips
 ----------------
 You can test your project using any specific combination of python, django and django cms.
 
-For example ``tox -epy3.7-django30-cms37`` runs the tests on python 3.7, Django 3.0 and django CMS 3.7.
+For example ``tox -py37-django30-cms37`` runs the tests on python 3.7, Django 3.0 and django CMS 3.7.
 
 
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