doc: fix broken link for crate::os::linux::raw::stat

Fixing:

 Documenting std v0.0.0 (/checkout/library/std)
error: `self::os::linux::raw::stat` is both a struct and a function
  --> library/std/src/os/linux/fs.rs:23:19
   |
23 |     /// [`stat`]: crate::os::linux::raw::stat
   |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^ ambiguous link
   |
   = note: `-D broken-intra-doc-links` implied by `-D warnings`
help: to link to the struct, prefix with `struct@`
   |
23 |     /// [`stat`]: struct@self::os::linux::raw::stat
   |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
help: to link to the function, add parentheses
   |
23 |     /// [`stat`]: self::os::linux::raw::stat()
   |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to previous error

error: could not document `std`

Signed-off-by: Tom Eccles <tom.eccles@codethink.co.uk>

library/std/src/os/linux/fs.rs

@@ -20,7 +20,7 @@ pub trait MetadataExt {
     /// Unix platforms. The `os::unix::fs::MetadataExt` trait contains the
     /// cross-Unix abstractions contained within the raw stat.
     ///
-    /// [`stat`]: crate::os::linux::raw::stat
+    /// [`stat`]: struct@crate::os::linux::raw::stat
     ///
     /// # Examples
     ///