alloc: remove repeated word in comment

Linux's `checkpatch.pl` reports:

```txt
rust-lang#42544: FILE: rust/alloc/vec/mod.rs:2692:
WARNING: Possible repeated word: 'to'
+            // - Elements are :Copy so it's OK to to copy them, without doing
```

Signed-off-by: Miguel Ojeda <[email protected]>

Author: ojeda

library/alloc/src/vec/mod.rs

@@ -2470,7 +2470,7 @@ impl<T: Copy, A: Allocator> ExtendFromWithinSpec for Vec<T, A> {
             // SAFETY:
             // - Both pointers are created from unique slice references (`&mut [_]`)
             //   so they are valid and do not overlap.
-            // - Elements are :Copy so it's OK to to copy them, without doing
+            // - Elements are :Copy so it's OK to copy them, without doing
             //   anything with the original values
             // - `count` is equal to the len of `source`, so source is valid for
             //   `count` reads