docs(readme): Update example to prefer DocumentMut

[slivingston]

Thanks for these great crates! I noticed that Document is deprecated but is used in the example of the toml_edit/README. This pull request updates it to DocumentMut.

Note that the similar example at the top of lib.rs already uses DocumentMut, so https://docs.rs/toml_edit/latest/toml_edit/#example is good. However, the README appears in the crates.io page. Thus this pull request is motivated.

[coveralls]

Pull Request Test Coverage Report for Build 12841135987

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 67.983%

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Totals
Change from base Build 12696320538:	0.0%
Covered Lines:	3752
Relevant Lines:	5519

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💛 - Coveralls

[epage]

Thanks!