Fix minor std::thread documentation typo

callers of spawn_unchecked() need to make sure that the thread
not outlive references in the passed closure, not the other way around.

Author: marcelo-gonzalez

std/src/thread/mod.rs

@@ -412,9 +412,9 @@ impl Builder {
     ///
     /// # Safety
     ///
-    /// The caller has to ensure that no references in the supplied thread closure
-    /// or its return type can outlive the spawned thread's lifetime. This can be
-    /// guaranteed in two ways:
+    /// The caller has to ensure that the spawned thread does not outlive any
+    /// references in the supplied thread closure and its return type.
+    /// This can be guaranteed in two ways:
     ///
     /// - ensure that [`join`][`JoinHandle::join`] is called before any referenced
     /// data is dropped