Remove explicit lifetime

clippy emits:

 error: the following explicit lifetimes could be elided: 'a

As suggested, remove the explicit lifetime.

Author: tcharding

src/lib.rs

@@ -764,7 +764,7 @@ mod prelude {
         impl<T> Mutex<T> {
             pub fn new(inner: T) -> Mutex<T> { Mutex { inner: RefCell::new(inner) } }
 
-            pub fn lock<'a>(&'a self) -> LockResult<MutexGuard<'a, T>> {
+            pub fn lock(&self) -> LockResult<MutexGuard<'_, T>> {
                 Ok(MutexGuard { lock: self.inner.borrow_mut() })
             }
         }