docs/Synchronized.md spelling (#1523)

Summary:
removed double of word 'lock'

Pull Request resolved: https://github.com/facebook/folly/pull/1523

Reviewed By: yfeldblum

Differential Revision: D26367154

Pulled By: Orvid

fbshipit-source-id: 586647d28b9ad002ee975450426d2da967dc638e

folly/docs/Synchronized.md

@@ -202,7 +202,7 @@ locked. This technique avoids the need to lock both mutexes at
 the same time. Mutexes are not copied or moved.
 
 The move assignment operator assumes the source object is a true
-rvalue and does lock lock the source mutex. It moves the source
+rvalue and does lock the source mutex. It moves the source
 data into the destination data with the destination mutex locked.
 
 `swap` acquires locks on both mutexes in increasing order of