Fix typo in Chapter 15 Section 05.

[ZiHao256]

Fix typo in Chapter 15 Section 05. changing:
"Rc<T> enables multiple owners of the same data; Box<T> and RefCell<T> have single owners ."
to
"Rc<T> enables multiple owners of the same data; Box<T> and RefCell<T> have single owner."

[carols10cents]

Sorry, this is not typo and is correct as-is. "Box<T> and RefCell<T>" is plural, so the correct match of the phrase is "single owners". Another way to think about this is replace "Box<T> and RefCell<T>" with the plural pronoun "they", which would make this sentence "they have single owners." which is grammatically correct. Thanks though!