Use more realistic example for thread builder

Stack size of 10 **bytes** does not make any sense: the minimal possible stack size is greater anyway.

src/libstd/thread/mod.rs

@@ -269,7 +269,7 @@ impl Builder {
     ///
     /// let builder = thread::Builder::new()
     ///                               .name("foo".into())
-    ///                               .stack_size(10);
+    ///                               .stack_size(32 * 1024);
     ///
     /// let handler = builder.spawn(|| {
     ///     // thread code