doc: follow idiom in code snippet

src/doc/reference.md

@@ -2762,7 +2762,7 @@ The following expressions are equivalent.
 let x = std::ops::Range {start: 0, end: 10};
 let y = 0..10;
 
-assert_eq!(x,y);
+assert_eq!(x, y);
 ```
 
 ### Unary operator expressions