Fix outdated default rust edition documentation

[hkennyv]

fixes #8951

this is my first PR to cargo, so please LMK if there's anything else I need to do :)

[rust-highfive]

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[ehuss]

The default if not specified is 2015. The default for cargo new is the latest edition (2018), it will explicitly place edition = "2018" in the manifest to override that default. But this documentation is for how the manifest itself interprets the field.

[hkennyv]

Ahh I see, thanks for the clarification @ehuss. I was confusing this with the default for cargo new!