BUG: stats: Fix ValueError message in chi2_contingency.

chi2_contingency raises a ValueError when a zero occurs in the
table of expected frequencies.  The exception message includes
the position of the zero, but the code that figured out that
position was incorrect.
Merge pull request #3891 from WarrenWeckesser/bug-chi2conting-zeropos

BUG: stats: Fix ValueError message in chi2_contingency.