Fixed Typo

[eltociear]

Pull Request Checklist

lenght -> length

Below is a simple checklist but please do not hesitate to ask for assistance!

• Fork, clone, and checkout the newest version of the code
• Create a new branch
• Make necessary code changes
• Install black (i.e., python -m pip install black or conda install -c conda-forge black)
• Install flake8 (i.e., python -m pip install flake8 or conda install -c conda-forge flake8)
• Install pytest-cov (i.e., python -m pip install pytest-cov or conda install -c conda-forge pytest-cov)
• Run black . in the root stumpy directory
• Run flake8 . in the root stumpy directory
• Run ./setup.sh && ./test.sh in the root stumpy directory
• Reference a Github issue (and create one if one doesn't already exist)

[seanlaw]

@eltociear Thank you for fixing this typo!