fix docs for inherent str constructors

[m4rch3n1ng]

related to #131114

when implementing inherent str constructors in #136517, i forgot to change the docs, so the code examples still imported the std::str module and used the constructor from there, instead of using "itself" (the inherent constructor).

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[Amanieu]

@bors r+ rollup

[bors]

📌 Commit 345c313 has been approved by Amanieu

It is now in the queue for this repository.