Fixed a small typo

Author: tivervac

src/doc/guide.md

@@ -4152,7 +4152,7 @@ We've made a struct that represents a circle. We then write an `impl` block,
 and inside it, define a method, `area`. Methods take a  special first
 parameter, `&self`. There are three variants: `self`, `&self`, and `&mut self`.
 You can think of this first parameter as being the `x` in `x.foo()`. The three
-variants correspond to the three kinds of thing `x` could be: `self` if it's
+variants correspond to the three kinds of things `x` could be: `self` if it's
 just a value on the stack, `&self` if it's a reference, and `&mut self` if it's
 a mutable reference. We should default to using `&self`, as it's the most
 common.