expand tox backend example
diff --git a/docs/contributing.rst b/docs/contributing.rst
index 3710abe..4bb1461 100644
--- a/docs/contributing.rst
+++ b/docs/contributing.rst
@@ -258,7 +258,7 @@
cryptography supports. Explicit backend selection can be done via the
``--backend`` flag. This flag should be passed to ``py.test`` with a comma
delimited list of backend names. To use it with ``tox`` you must pass it as
-``-- --backend``.
+``tox -- --backend=openssl``.
Building Documentation
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