fix typos in cranelift-frontend/src/lib.rs
//!
//! This API has been designed to help you translate your mutable variables into
//! [`SSA`](https://en.wikipedia.org/wiki/Static_single_assignment_form) form.
-//! [`use_var`](struct.FunctionBuilder.html#method.use_var) will returns the Cranelift IR value
-//! that corresponds to your mutable variable at a precise point in the program. However, you know
+//! [`use_var`](struct.FunctionBuilder.html#method.use_var) will return the Cranelift IR value
+//! that corresponds to your mutable variable at a precise point in the program. However, if you know
//! beforehand that one of your variables is defined only once, for instance if it is the result
//! of an intermediate expression in an expression-based language, then you can translate it
//! directly by the Cranelift IR value returned by the instruction builder. Using the
//! beforehand if a variable is immutable or not).
//!
//! The moral is that you should use these three functions to handle all your mutable variables,
-//! even those that are not present in the source code but artefacts of the translation. It is up
+//! even those that are not present in the source code but artifacts of the translation. It is up
//! to you to keep a mapping between the mutable variables of your language and their `Variable`
//! index that is used by Cranelift. Caution: as the `Variable` is used by Cranelift to index an
//! array containing information about your mutable variables, when you create a new `Variable`
