Fix them doc examples some more

Apologies for the many attempts, my dev loop for this consists of editing on github, committing, and then waiting for the CI failure log to yell at me.

alloc/src/boxed.rs

@@ -842,7 +842,7 @@ impl<T, A: Allocator> Box<[T], A> {
     ///
     /// use std::alloc::System;
     ///
-    /// let mut values = Box::<[u32]>::try_new_uninit_slice_in(3, System)?;
+    /// let mut values = Box::<[u32], _>::try_new_uninit_slice_in(3, System)?;
     /// let values = unsafe {
     ///     // Deferred initialization:
     ///     values[0].as_mut_ptr().write(1);
@@ -885,7 +885,7 @@ impl<T, A: Allocator> Box<[T], A> {
     ///
     /// use std::alloc::System;
     ///
-    /// let values = Box::<[u32]>::try_new_zeroed_slice_in(3, System)?;
+    /// let values = Box::<[u32], _>::try_new_zeroed_slice_in(3, System)?;
     /// let values = unsafe { values.assume_init() };
     ///
     /// assert_eq!(*values, [0, 0, 0]);