macros: typo fix

spotted while reviewing the todo!macro docs

src/libcore/macros/mod.rs

@@ -551,7 +551,7 @@ macro_rules! unreachable {
 
 /// Indicates unimplemented code by panicking with a message of "not implemented".
 ///
-/// This allows the your code to type-check, which is useful if you are prototyping or
+/// This allows your code to type-check, which is useful if you are prototyping or
 /// implementing a trait that requires multiple methods which you don't plan of using all of.
 ///
 /// The difference between `unimplemented!` and [`todo!`](macro.todo.html) is that while `todo!`