Error message grammar/typo: 'and unstable location' -> 'an' unstable location'

[hunteke]

Rust learner here, diving into logging, and using my incorrect code. Very helpful and specific error message (OMG, thank you Rust devs!); just has a typo. A two-minute find-and-correct situation. The error message reads:

$ cargo build
   Compiling typo_error_msg v0.1.0 (file:///tmp/typo_error_msg)
error: use of unstable library feature 'rustc_private': this crate is being loaded from the sysroot, and unstable location; did you mean to load this crate from crates.io via `Cargo.toml` instead? (see issue #27812)

Note the ... sysroot, and unstable location; .... Should the and perhaps be an?

Steps to recreate:

1. cargo new --bin typo_error_msg
2. cd typo_error_msg
3. sed -i '1i #[macro_use] extern crate log;\n' src/main.rs
4. cargo build

[estebank]

Fixed in #45058.