Typo in RefCell's section (immutable -> mutable)

Location

https://doc.rust-lang.org/std/cell/index.html#refcellt

An immutable reference to a RefCell’s inner value (&T) can be obtained with borrow, and a mutable borrow (&mut T) can be obtained with borrow_mut. When these functions are called, they first verify that Rust’s borrow rules will be satisfied: any number of *immutable* borrows are allowed or a *single immutable* borrow is allowed, but never both. If a borrow is attempted that would violate these rules, the thread will panic.

Summary

any number of immutable borrows are allowed or a single mutable (immutable -> mutable) borrow is allowed, but never both

[compiler-errors]

This was fixed in #112206.