Fix noop_method_call warning in doctest

```
error: call to `.clone()` on a reference in this situation does nothing
  --> crossbeam-epoch/src/guard.rs:451:38
   |
14 |     let dummy = &epoch::unprotected().clone();
   |                                      ^^^^^^^^ help: remove this redundant call
   |
   = note: the type `Guard` does not implement `Clone`, so calling `clone` on `&Guard` copies the reference, which does not do anything and can be removed
note: the lint level is defined here
  --> crossbeam-epoch/src/guard.rs:438:9
   |
1  | #![deny(warnings, rust_2018_idioms)]
   |         ^^^^^^^^
   = note: `#[deny(noop_method_call)]` implied by `#[deny(warnings)]`
```

crossbeam-epoch/src/guard.rs

@@ -447,8 +447,8 @@ impl fmt::Debug for Guard {
 ///     // Load `a` without pinning the current thread.
 ///     a.load(Relaxed, epoch::unprotected());
 ///
-///     // It's possible to create more dummy guards by calling `clone()`.
-///     let dummy = &epoch::unprotected().clone();
+///     // It's possible to create more dummy guards.
+///     let dummy = epoch::unprotected();
 ///
 ///     dummy.defer(move || {
 ///         println!("This gets executed immediately.");