[MRG] Fix minor typo in doc
Could also be *When unit tests fail*?
@@ -79,7 +79,7 @@ to select tests based on their name. For instance,::
will run all :term:`common tests` for the ``LogisticRegression`` estimator.
-When a unit tests fail, the following tricks can make debugging easier:
+When a unit test fails, the following tricks can make debugging easier:
1. The command line argument ``pytest -l`` instructs pytest to print the local
variables when a failure occurs.
