fix troubleshooting blas rec to run in py3

doc/troubleshooting.txt

@@ -289,7 +289,7 @@ Theano/BLAS speed test:
 
 .. code-block:: bash
 
-    python `python -c "import os, theano; print os.path.dirname(theano.__file__)"`/misc/check_blas.py
+    python `python -c "import os, theano; print(os.path.dirname(theano.__file__))"`/misc/check_blas.py
 
 This will print a table with different versions of BLAS/numbers of
 threads on multiple CPUs and GPUs. It will also print some Theano/NumPy