docstrings: typos, clarity

[stragu]

Minor changes only in docstrings: capitalisation, hyphenation, "indexes" -> "indices", typos.

• tests added / passed
• Ensure all linting tests pass, see here for how to run them
• whatsnew entry

Apologies for not testing, as the changes are minor and only affect docstrings.
No whatsnew entry either because minor.

[MarcoGorelli]

both spellings are acceptable https://grammarist.com/usage/indexes-indices/

[mzeitlin11]

both spellings are acceptable https://grammarist.com/usage/indexes-indices/

Not sure if distinction is necessary, but from a pandas perspective I like the idea of

• Indexes: Multiple Index objects
• Indices: Values used to index into something

[stragu]

Thank you both for your comments. I amended the commit to not touch indices vs indexes.

[simonjayhawkins]

Thanks @stragu