Fix outdated reference in docstring (#1688)

[ezioruan]

• I have provided code that clearly demonstrates the bug and that only works correctly when applying this fix
• I have added suitable tests demonstrating a fixed bug or new/changed feature to the test suite in tests/
• I have properly documented new or changed features in the documentation or in the docstrings
• I have properly explained unusual or unexpected code in the comments around it

[coveralls]

Coverage remained the same at 84.789% when pulling 8d56979 on ezioruan:fix_1688 into 5853ff8 on Zulko:master.

[keikoro]

@ezioruan Letting you know we've updated our CI settings. If you could update your PR to incorporate the latest changes to the code base – rebase master, (force) push the branch you used to open this PR – that'd be great.

[ezioruan]

@ezioruan Letting you know we've updated our CI settings. If you could update your PR to incorporate the latest changes to the code base – rebase master, (force) push the branch you used to open this PR – that'd be great.
sure. Done

[mondeja]

Thanks!