Correct the error message to new filename.

[jwg4]

The file which needs to be manually edited now seems to be config_defaults.py._

• If this is a bugfix, I have provided code that clearly demonstrates the problem and that works when used with this PR
• [.] I have added a test to the test suite, if necessary
• [.] I have properly documented new or changed features in the documention, or the docstrings
• [.] I have properly documented unusual changes to the code in the comments around it
• I have made note of any breaking/backwards incompatible changes

[coveralls]

Coverage increased (+0.2%) to 65.005% when pulling aa3f30b on jwg4:patch-1 into 65f6b47 on Zulko:master.

[tburrows13]

Thanks