Rollup merge of rust-lang#28525 - Wallacoloo:book-4.8-double-more, r=steveklabnik

This is a simple grammar fix in which the previous author accidentally repeated a word when (s)he shouldn't have.

Author: Manishearth

src/doc/trpl/choosing-your-guarantees.md

@@ -321,7 +321,7 @@ there's a lot of concurrent access happening.
 
 # Composition
 
-A common gripe when reading Rust code is with types like `Rc<RefCell<Vec<T>>>` (or even more more
+A common gripe when reading Rust code is with types like `Rc<RefCell<Vec<T>>>` (or even more
 complicated compositions of such types). It's not always clear what the composition does, or why the
 author chose one like this (and when one should be using such a composition in one's own code)