docs: use intra-doc links for Vec::get(_mut)

Now that rust-lang#63351 is fixed, there's no reason not to.

Author: notriddle

library/alloc/src/vec/mod.rs

@@ -378,8 +378,8 @@ mod spec_extend;
 /// Currently, `Vec` does not guarantee the order in which elements are dropped.
 /// The order has changed in the past and may change again.
 ///
-/// [`get`]: ../../std/vec/struct.Vec.html#method.get
-/// [`get_mut`]: ../../std/vec/struct.Vec.html#method.get_mut
+/// [`get`]: slice::get
+/// [`get_mut`]: slice::get_mut
 /// [`String`]: crate::string::String
 /// [`&str`]: type@str
 /// [`shrink_to_fit`]: Vec::shrink_to_fit