Remove extra parenthesis (#1375)

CppCoreGuidelines.md

@@ -14774,7 +14774,7 @@ Flag all unnamed `lock_guard`s and `unique_lock`s.
 It should be obvious to a reader that the data is to be guarded and how. This decreases the chance of the wrong mutex being locked, or the mutex not being locked.
 
 Using a `synchronized_value<T>` ensures that the data has a mutex, and the right mutex is locked when the data is accessed.
-See the [WG21 proposal](http://wg21.link/p0290)) to add `synchronized_value` to a future TS or revision of the C++ standard.
+See the [WG21 proposal](http://wg21.link/p0290) to add `synchronized_value` to a future TS or revision of the C++ standard.
 
 ##### Example