typo fix for `nullable()` documentation
I guess..
@@ -1316,7 +1316,7 @@ pub trait QueryDsl: Sized {
/// Coerce the SQL type of the select clause to it's nullable equivalent.
///
- /// This is use full for writing queries that contain subselects on non null
+ /// This is useful for writing queries that contain subselects on non null
/// fields comparing them to nullable fields.
/// ```rust
/// # include!("../doctest_setup.rs");
