`File::set_times`: Remove `write(true)` from the example so it works on directories

Inspired by https://github.com/rust-lang/rust/issues/123883 .
diff --git a/library/std/src/fs.rs b/library/std/src/fs.rs
index 72ad7c24..08976ae 100644
--- a/library/std/src/fs.rs
+++ b/library/std/src/fs.rs
@@ -1128,7 +1128,7 @@ pub fn set_permissions(&self, perm: Permissions) -> io::Result<()> {
     ///     use std::fs::{self, File, FileTimes};
     ///
     ///     let src = fs::metadata("src")?;
-    ///     let dest = File::options().write(true).open("dest")?;
+    ///     let dest = File::open("dest")?;
     ///     let times = FileTimes::new()
     ///         .set_accessed(src.accessed()?)
     ///         .set_modified(src.modified()?);