Update ownership.md

Opening sentence was confusing and something cannot be "one of the most unique" (it either is or is not unique).

Author: Michael Tiller

src/doc/book/ownership.md

@@ -1,7 +1,7 @@
 % Ownership
 
-This guide is one of three presenting Rust’s ownership system. This is one of
-Rust’s most unique and compelling features, with which Rust developers should
+This is the first of three sections presenting Rust’s ownership system. This is one of
+Rust’s most distinct and compelling features, with which Rust developers should
 become quite acquainted. Ownership is how Rust achieves its largest goal,
 memory safety. There are a few distinct concepts, each with its own
 chapter: