Path::with_extension: show that it adds an extension where one did not exist

I think the times I encountered this, I had to check first if files without extensions were added, since all examples only had files with existing extensions.

Also, this replaced example already has a similar example below.

Author: tshepang

library/std/src/path.rs

@@ -2741,7 +2741,7 @@ impl Path {
     /// ```
     /// use std::path::{Path, PathBuf};
     ///
-    /// let path = Path::new("foo.rs");
+    /// let path = Path::new("foo");
     /// assert_eq!(path.with_extension("txt"), PathBuf::from("foo.txt"));
     ///
     /// let path = Path::new("foo.tar.gz");