Remove spurious backtick detected by rustdoc::unescaped_backticks

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

Author: ojeda

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"]