Fix typo in precent-encoding documentation
//!
//! This crate delibarately does not provide many different sets.
//! Users should consider in what context the encoded string will be used,
-//! real relevant specifications, and define their own set.
+//! read relevant specifications, and define their own set.
//! This is done by using the `add` method of an existing set.
//!
//! # Examples
