Fixed small spelling error.

Checklist

• My pull request has a clear and explanatory title.
• If necessary, my code is vectorized.
• I added appropriate unit tests.
• I made sure the code passes all unit tests. (refer to comment below)
• My PR follows PEP8 guidelines. (refer to comment below)
• My PR follows geomstats coding style and API.
• My code is properly documented and I made sure the documentation renders properly. (Link)
• I linked to issues and PRs that are relevant to this PR.

Description

Issue

Additional context