MNT: correct _replacer docstring

[rcomer]

PR summary

sanitize_sequence does not convert everything to a sequence. It specifically only converts MappingView objects. So, for example, it you pass an int to _replacer you get an int back.

I had reason to worry about this for my proposed changes in #29152

PR checklist

• "closes #0000" is in the body of the PR description to link the related issue
• new and changed code is tested
• Plotting related features are demonstrated in an example
• New Features and API Changes are noted with a directive and release note
• Documentation complies with general and docstring guidelines

[story645]

Oops, this may have needed two reviews since it's docs in code vs docs in docs 🤦‍♀️

[rcomer]

Thanks @story645! I think this falls under

not fundamental and can easily be reverted at any time in the future

https://matplotlib.org/devdocs/devel/pr_guide.html#merging

So I reckon we're good 🙂