Fix article in Result.map and Result.map_err documentation

[dbrgn]

Fix usage of wrong article in Result docs.

This is my first rust PR, if something about the process is wrong let me know.

As this is a documentation change, I believe the correct highfive line to use is the following:

r? @steveklabnik

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @steveklabnik (or someone else) soon.

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[steveklabnik]

Nope, this is perfect! Thanks so much!

@bors: r+ rollup

[bors]

📌 Commit 9b5b0cd has been approved by steveklabnik