doc: fix a typo

Summary:
s/synchromization/synchronization/
Closes https://github.com/facebook/rocksdb/pull/3583

Differential Revision: D7276596

Pulled By: sagar0

fbshipit-source-id: 552ec6d6935f642e1a3a7c552de6c94441ac50e0

db/memtable.h

@@ -64,7 +64,7 @@ struct MemTablePostProcessInfo {
 };
 
 // Note:  Many of the methods in this class have comments indicating that
-// external synchromization is required as these methods are not thread-safe.
+// external synchronization is required as these methods are not thread-safe.
 // It is up to higher layers of code to decide how to prevent concurrent
 // invokation of these methods.  This is usually done by acquiring either
 // the db mutex or the single writer thread.