TST: Fix error in assert_produces_warning.

When I copied this over and sent to @cpcloud,
I thought the indentation was wrong and changed it.
Turned out I was wrong -- sorry about that!

Now all the doctests pass...
Merge pull request #3927 from jtratner/fix-indentation-error-in-assert-produces-warning

TST: Fix error in assert_produces_warning.