Fix panic message referencing the wrong builder extension

src/platform_impl/linux/mod.rs

@@ -597,7 +597,7 @@ impl<T: 'static> EventLoop<T> {
                 "Initializing the event loop outside of the main thread is a significant \
                  cross-platform compatibility hazard. If you absolutely need to create an \
                  EventLoop on a different thread, you can use the \
-                 `EventLoopBuilderExtWindows::any_thread` function."
+                 `EventLoopBuilderExtUnix::any_thread` function."
             );
         }