doc: missing quote in keyword Send

Author: yorkie

src/doc/tutorial.md

@@ -979,7 +979,7 @@ let mut b = Foo { x: 5, y: box 10 };
 b.x = 10;
 ~~~~
 
-If an object doesn't contain any non-Send types, it consists of a single
+If an object doesn't contain any non-`Send` types, it consists of a single
 ownership tree and is itself given the `Send` trait which allows it to be sent
 between tasks. Custom destructors can only be implemented directly on types
 that are `Send`, but non-`Send` types can still *contain* types with custom