BUG Fix typo in InfeasibleTestError exception string

[emorisse]

• closes #xxxx
• [? ] tests added / passed.
• [x ] code/documentation is well formatted.
• [x ] properly formatted commit message. See
  NumPy's guide.

I've updated statsmodels/tsa/stattools.py to correct a minor typo (compute -> computed) for the InfeasibleTestError thrown within grangercausalitytests().

I'm not sure how to write a test to check a string but have included a test that will throw the exception in question to verify the string.

[bashtage]

Thanks.