Remove spurious backticks detected by rustdoc::unescaped_backticks

There are only 3 cases across the crates rendered in the website (`core`,
`alloc`, `std`, `proc_macro` and `test`), and they are all in `core`.

Clean them up, so that the lint can be enabled in the next commit.

Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

library/core/src/ops/deref.rs

@@ -282,7 +282,7 @@ impl<T: ?Sized> DerefMut for &mut T {
 /// FIXME(deref_patterns): The precise semantics are undecided; the rough idea is that
 /// successive calls to `deref`/`deref_mut` without intermediate mutation should be
 /// idempotent, in the sense that they return the same value as far as pattern-matching
-/// is concerned. Calls to `deref`/`deref_mut`` must leave the pointer itself likewise
+/// is concerned. Calls to `deref`/`deref_mut` must leave the pointer itself likewise
 /// unchanged.
 #[unstable(feature = "deref_pure_trait", issue = "87121")]
 #[lang = "deref_pure"]

library/core/src/ptr/mod.rs

@@ -786,7 +786,7 @@ where
 ///
 /// The caller must also ensure that the memory the pointer (non-transitively) points to is never
 /// written to (except inside an `UnsafeCell`) using this pointer or any pointer derived from it. If
-/// you need to mutate the pointee, use [`from_mut`]`. Specifically, to turn a mutable reference `m:
+/// you need to mutate the pointee, use [`from_mut`]. Specifically, to turn a mutable reference `m:
 /// &mut T` into `*const T`, prefer `from_mut(m).cast_const()` to obtain a pointer that can later be
 /// used for mutation.
 ///

library/core/src/slice/mod.rs

@@ -3452,8 +3452,8 @@ impl<T> [T] {
     /// elements of the slice move to the end while the last `k` elements move
     /// to the front.
     ///
-    /// After calling `rotate_right`, the element previously at index `self.len()
-    /// - k` will become the first element in the slice.
+    /// After calling `rotate_right`, the element previously at index
+    /// `self.len() - k` will become the first element in the slice.
     ///
     /// # Panics
     ///