Fix markdown formatting error in docs for folly Synchronized

Summary: Fixing an incorrectly formatted link in https://github.com/facebook/folly/blob/main/folly/docs/Synchronized.md.

Reviewed By: yfeldblum

Differential Revision: D32227087

fbshipit-source-id: 773ed8f1a0651a177fd306b63b514a917a8934bd

folly/docs/Synchronized.md

@@ -711,7 +711,7 @@ locking of two objects and offering their innards.  It returns a
 ```
 
 Note that C++ 17 introduces
-(structured binding syntax)[(http://wg21.link/P0144r2)]
+[structured binding syntax](http://wg21.link/P0144r2)
 which will make the returned tuple more convenient to use:
 
 ``` Cpp