runtime: fix typo in runtime builder docs
@@ -818,7 +818,7 @@ impl Builder {
///
/// This configuration option is considered a workaround for the LIFO
/// slot not being stealable. When the slot becomes stealable, we will
- /// revisit whther or not this option is necessary. See
+ /// revisit whether or not this option is necessary. See
/// tokio-rs/tokio#4941.
///
/// # Examples
