book: fix sentence

Author: tshepang

src/doc/trpl/documentation.md

@@ -33,8 +33,10 @@ pub fn new(value: T) -> Rc<T> {
 ```
 
 This code generates documentation that looks [like this][rc-new]. I've left the
-implementation out, with a regular comment in its place. That's the first thing
-to notice about this annotation: it uses `///`, instead of `//`. The triple slash
+implementation out, with a regular comment in its place.
+
+The first thing to notice about this annotation is that it uses
+`///` instead of `//`. The triple slash
 indicates a documentation comment.
 
 Documentation comments are written in Markdown.