Docs: Change "statements" to "expressions" on match

I apt to use `expressions` over `statements`, because `match` is a expression in essence, though it can become a statement when followed a semicolon.

src/doc/book/patterns.md

@@ -1,7 +1,7 @@
 % Patterns
 
 Patterns are quite common in Rust. We use them in [variable
-bindings][bindings], [match statements][match], and other places, too. Let’s go
+bindings][bindings], [match expressions][match], and other places, too. Let’s go
 on a whirlwind tour of all of the things patterns can do!
 
 [bindings]: variable-bindings.html