rust: task: remove redundant explicit link

Starting with Rust 1.73.0, `rustdoc` detects redundant explicit
links with its new lint `redundant_explicit_links` [1]:

    error: redundant explicit link target
      --> rust/kernel/task.rs:85:21
       |
    85 |     /// [`current`](crate::current) macro because it is safe.
       |          ---------  ^^^^^^^^^^^^^^ explicit target is redundant
       |          |
       |          because label contains path that resolves to same destination
       |
       = note: when a link's destination is not specified,
               the label is used to resolve intra-doc links
       = note: `-D rustdoc::redundant-explicit-links` implied by `-D warnings`
    help: remove explicit link target
       |
    85 |     /// [`current`] macro because it is safe.

In order to avoid the warning in the compiler upgrade commit,
make it an intra-doc link as the tool suggests.

Link: rust-lang/rust#113167 [1]
Reviewed-by: Finn Behrens <[email protected]>
Reviewed-by: Alice Ryhl <[email protected]>
Reviewed-by: Martin Rodriguez Reboredo <[email protected]>
Reviewed-by: Vincenzo Palazzo <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Miguel Ojeda <[email protected]>

rust/kernel/task.rs

@@ -82,7 +82,7 @@ impl Task {
     /// Returns a task reference for the currently executing task/thread.
     ///
     /// The recommended way to get the current task/thread is to use the
-    /// [`current`](crate::current) macro because it is safe.
+    /// [`current`] macro because it is safe.
     ///
     /// # Safety
     ///