Minor update to test run instructions in doc/troubleshooting.txt

doc/troubleshooting.txt

@@ -107,13 +107,14 @@ Also, check the following outputs :
     theano.__version__
 
 
-Once you have installed Theano, you should run the test suite.
+Once you have installed Theano, you should run the test suite in the ``tests`` directory.
 
 .. code-block:: bash
 
     python -c "import numpy; numpy.test()"
     python -c "import scipy; scipy.test()"
-    THEANO_FLAGS=''; python -c "import theano; theano.test()"
+    pip install pytest
+    THEANO_FLAGS='' pytest tests/
 
 All Theano tests should pass (skipped tests and known failures are normal). If
 some test fails on your machine, you are encouraged to tell us what went